An issue that is often raised when discussing aquathermal energy is: “What impact will this technology have on the waterbody’s flora and fauna?”

When heat is extracted from surface water, it is returned at a lower temperature in Winter (heating season) and a higher temperature in Summer (cooling season). This temperature change is known as the thermal impact.

In an open loop system, a so called ‘cold plume or warm plume’ is created and in a closed loop system, the water around the heat exchanger in the waterbody will also cool down. As the water moves away from the discharge point or heat exchanger it starts mixing with the water in the waterbody and the temperature difference will even out a short distance away from the discharge point or the heat exchanger.

Figure of a cold plume (blue color) in an open loop aquathermal system being discharged in a waterbody where mixing starts to occur

However when waterbodies are cooled or heated too much this has an impact on plant and animal metabolism as already can be observed seasonally (winter, spring, summer, autumn).  As a result, significant cold discharges can theoretically have a wide range of ecological effects, both on individual species and on species groups. To what extent these effects occur can vary greatly depending on the specific waterbody, the size and location of the system that is installed and the positioning of the system discharge (if applicable). While in depth knowledge of the effect is still being studied some examples of effects that can occur in poorly designed systems include:

• For fish, macrofauna, phytoplankton and plants a lower water temperature can reduce productivity, growth, and development, especially in spring.
• For fish, the spawn season can also be altered because of a change in temperature.
• If a thermal plume takes place across a significant part of the cross section in a river, it can form a migration barrier for fish.
• It is generally assumed that a cooling effect in the waterbody creates less impact/stress than a warming effect (when for examples buildings are cooled with aquathermal systems).
• Cooling can be even beneficial to prevent the growth of algae in summertime.

Ensuring harmless impact

To make sure aquathermal systems don’t create a harmful effect in the waterbody, systems need to be designed with respect to boundaries of the waterbody’s ecosystem. To do so, it is therefore important that a feasibility study is carried out to analyse the impact of an aquathermal system before it is built. These feasibility studies should include in depth detail on parameters such as the effect of regeneration, the local impact in 3D, etc. A guideline of what a feasibility study of the thermal impact should include is currently created through the Interreg North Sea project WaterWarmth.

When detailed thermal simulations are carried out, the temperature impact can be analysed and checked to ensure it complies with the national or regional regulations. Clear local regulations for aquathermal systems are however not in place in EU Member States at this moment. The AquaCOM project, an initiative to empower energy communities in North-West Europe to use aquathermal energy, is assessing existing regulations and how they could be optimised in the future.

Thermal simulations to evaluate the impact of aquathermal systems should include the effect of regeneration. A waterbody acts like a thermal battery, every time heat is extracted, the surroundings (sun, air, ground) will recharge the thermal battery.