Ecological impacts of thermal energy from surface water : cooling and filtration effects

Thermal Energy from Surface Water (TES) is a promising climate-neutral heating solution in the Netherlands, with the potential to meet around 40% of residential heat demand. However, its ecological impacts remain poorly understood, particularly in relation to the mechanical filtration of aquatic organisms. This study uses ecosystem modelling to assess ecological effects of TES-induced cooling and filtration in a shallow lake with a high residence time. A range of scenarios was simulated, using varying temperature differentials (ΔT = 5-10 °C lower than the existing surface water) and flow rates (Q = 75-300 m³/h), and focusing on changes in chlorophyll-a concentrations and primary production. Results show that TES operation substantially reduces phytoplankton biomass and productivity, with declines of up to 56.1% and 65.9% respectively during the growing season. Filtration further amplified these effects by ~5-6%, depending on the intensity of the filter (e.g., chlorophyll-a: -50.4% without filter vs. -56.0% with filter (21% mortality)) because it removed planktonic organisms and disrupted trophic dynamics of planktonic organisms, particularly under stagnant conditions where the daily filtered volume exceeded 1% of the waterbody. Cooling effects were strong (seasonal water-temperature reductions up to 37.3%), and biotic recovery lagged beyond operation: chlorophyll-a and primary production remained ~15% lower into late autumn. In our worst-case scenario (ΔT 10, Q 150) the filtered volume (4-16% day⁻¹) exceeded precautionary practice (~1% day⁻¹), emphasising the need to co-optimize ΔT and Q, limit filtration intensity, and meet intake design criteria (e.g., ≤0.15 m s⁻¹, mesh ≥1.5 mm). While TES holds substantial potential, its deployment must be guided by ecologically informed criteria to mitigate impacts on aquatic ecosystems.