Focus on heat transfer fluids – technology, environment and approval
The use of wastewater for heat recovery is becoming increasingly important. It offers great potential for the heat transition, especially in urban areas. Heat exchangers can be used to extract thermal energy from the wastewater stream and make it usable for heating systems. BETREM GmbH is working in cooperation with RPTU Kaiserslautern within the INTERREG NWE project ‘ResNRJwater’ on the safe and sustainable implementation of such systems. Ann-Kathrin Kohmann and Manuel Schriever, both working at the project partner BETREM GmbH, have compiled the following results:
The role of the heat transfer fluid
When designing a heat exchanger system, choosing a suitable heat transfer medium is key. Water is the most environmentally friendly option with good thermal properties, but there is a risk of freezing or possible corrosion. Water-glycol mixtures are also commonly chosen, as they can be used in the heat exchanger system even at lower temperatures. However, glycol is classified as hazardous to water (WHC 1). A leak due to material defects or faulty connections can cause environmental damage.
Alternative heat transfer fluids
The following alternatives to glycol were
investigated:
- Special waters with biodegradable corrosion
protection
- Bioethanol or potassium formate-based fluids
- Water with additives such as EndoTherm to increase
efficiency
All options were evaluated in an assessment matrix according to criteria such as thermal conductivity, viscosity, environmental impact, availability and safety.
Selecting the right heat transfer fluid requires a holistic approach. Technical requirements, environmental compatibility and operational safety must be evaluated together and specifically tailored to the respective application.
Technical leakage protection
The use of water-polluting heat transfer fluids in wastewater heat exchangers can have an environmental impact in the event of a leak. In order to detect leaks early and minimise the volume of leakage from the installed heat exchangers and thus also the environmental risks, the following protective measures are possible:
- Pressure sensors and leak detectors
- Double-walled pipes with gap monitoring
- Siphons to limit the impact in the event of a leak
- Check valves to prevent the system from running dry
The appropriate protective measure must be specifically tailored to the heat transfer fluid used, the specific system design and the respective location.
Legal assessment
The use of water-polluting heat transfer fluids is subject to the provisions of the German Water Resources Act (WHG), the Waste Water Ordinance (AbwV) and the Ordinance on Installations for Handling Substances Hazardous to Water (AwSV). The mandatory classification into various water hazard classes is the responsibility of the operators. Suitable protective measures must also be taken for glycol. Whether a plant requires a permit is decided on a case-by-case basis. The applicable legal framework provides a central basis for the planning and safe operation of such plants.
Conclusion
Heat recovery from wastewater is a groundbreaking contribution to the decarbonisation of heat supply. To ensure that it can be implemented safely and in an environmentally friendly manner, heat transfer fluids must be carefully selected, integrating protective technical measures while taking into account approval requirements. The assessment matrix that has been developed will support operators, planners and authorities in making qualified decisions – for safe and sustainable plant operation.
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