Thermal Energy from Surface Waters

Abstract

Research towards aquathermal energy solutions is still in a pioneering phase. Especially in urban environments, these solutions could potentially form the missing connection between heating supply and demand. This study focuses on the application of these solutions in the city canal waters in the largest city of the Netherlands, Amsterdam . Currently, these so-called Surface Water Heat (SWH) systems have not been implemented at all in the inner city canals of Amsterdam. One of the reasons for this can be found in a lack of knowledge about natural water temperature variations in these types of waters and the unknown thermal effects related to SWH systems. This study is an answer to this knowledge gap and an attempt is made to provide insight in the thermal effects of SWH on a city canal. The Amsterdam city canals form a unique water system in open connection with larger waterways and they are part of a hydrodynamically complex interaction between sea and the river Rhine. Until date, such a study towards SWH system effects, not being restricted to the boundaries of the respective water body, is scarce or even non-existent.\ The case study location is situated at the Jacob van Lennepkanaal in Amsterdam, where a collective of civilians has recently started the exploratory phase for installation of the first SWH city canal system for heating of their own district. To respond to this societal development, this specific case study location was chosen. To be able to model the SWH system for this neighborhood, the first part of this thesis focuses on system layout and related system characteristics. It was found that open-loop SWH systems are often applied at district level. Secondly, a reference surface water temperature model without SWH systems was built and validated upon meteorological forcing with local water temperature measurements. The third part of the followed method was to apply the modelled SWH system to this validated three-dimensional thermodynamic model. For this purpose, the Delft3D model was chosen, which computes the RANS equations under the hydrostatic assumption. Several scenarios were calculated with varying temperature difference through the SWH heat exchanger and varying distance between system intake and outfall. The modelling results for the Jacob van Lennepkanaal case show that SWH systems used for heating purposes, thereby cooling the surface water, produce a negatively buoyant flow, causing the discharged cold water plume to sink to the bottom. Existing thermal stratification in the city canals is enhanced and hydrodynamic mixing processes are not strong enough to disturb this stratification built-up. Further away along the plume, in the far-field, a combination of buoyant spreading and the process of passive diffusion mix the water column and the temperature decrease, induced by the SWH system, becomes uniformly spread over the entire water column. Currently, steady state one-dimensional models are often used for assessment of SWH thermal effects. It was demonstrated that these models are useful and accurate, provided that the right heat exchange coefficient is used and near-field mixing at the outfall is represented correctly. These models can be used to study for example the time-averaged extent of the cold water plume, but fail to provide insight in time-varying and spatial characteristics of the canal. Local changes in bed level height, attraction of ambient water by the intake and intersections with lateral canals also influence the thermal profile. For a study towards these effects in greater detail, three-dimensional modelling or 2DV-modelling are preferred. Also, in case of a study towards application of multiple systems, these are the models of choice.