The effect of thermal energy recovery on the ecology of a small, slow flowing fresh water ecosystem

Abstract

The Netherlands is searching for alternative technologies for heating the built environment to reduce gas mining activities under the province of Groningen and to reduce the emission of greenhouse gasses. Recovery of thermal energy from small (maximum 20 m wide and 3 m deep), slow flowing (maximum 10 m3/s) fresh surface water ecosystems (stream) with Hydro Thermal Energy (HTE) can be such a technology.
This thermal energy recovery results in cold water discharge that affects the temperature of a stream which can subsequently influence the biological activity, and thus the ecology in a stream both positively and negatively. Since biodiversity loss is also a major global environmental problem one should avoid the negative effects and enhance the positive effects on the ecology. Therefore, it is important to know to what extend the ecology of a stream is affected by thermal energy recovery with HTE. To come to this knowledge a dynamic model, with a selection of thermal influence characteristics and ecological indicators, has been built to generate insight in the annual interaction between HTE and the stream ecology.
The thermal characteristics describe the whole of thermal energy demand and the thermal influence on the stream. The incoming thermal energy in the stream is the first characteristic. With the extracted thermal energy (cold water discharge) as the central characteristic we determined the resulting temperature of the stream; the longitudinal range of the thermal influence along the stream and the available thermal energy for the built environment.
The ecological indicators were selected because they have a direct quantifiable relation with (changing) temperature, quantifying literature was available and they cover multiple trophic levels. The general thermal water quality score was based on general water quality classifications as a function of the temperature of a stream. The fish spawning potential is based on a time (over a year) window and a temperature window at which a fish species, present in the Dutch fresh surface water, is able to spawn. With a Specie Sensitivity Distribution based on temperature, the Potentially Not Occurring Fraction of freshwater bivalves could be determined.
A hypothetical case based on a stream called the Goorloop near Helmond, the Netherlands, with two HTE-system configurations, has been applied in this model. The outcome, which varies considerably due to variation in the flow of the stream, is the size of the thermal influence:
a) The temperature decreases with 1.15 up to 18.1 ◦C
b) The longitudinal range is 0.5 up to 10 km
c) The available thermal energy for the built environment is 22 up to 44 TJ
By calculating the status of these ecological indicators with the daily temperature before and after (resulting temperature) HTE, the annual effect of cold water discharge by HTE on the ecology is:
a) The general thermal water quality score increased with 1%;
b) The fish spawning potential decreased with 1 up to 24%;
c) The potentially not occurring fraction of bivalves increased with 35 up to 108%
This research can form the base for future research for both specific cases and generic guidelines for thermal energy recovery from a stream with HTE. The model can be developed further with additional ecological indicators and present stressors, a more accurate longitudinal range and more advanced thermal energy recovery strategies. Besides, more streams and longer time frames could be applied.
We conclude that HTE affects the ecology of a stream both positively (early summer) and negatively (late winter and spring) and that the developed model can help identify the moments in time at which these effects occur.

Date of Award	26 Oct 2020
Original language	English
Supervisor	Wilfried Ivens (Examiner) & Stef Boesten (Co-assessor)