21 Jun 2016

Improved technologies remove risk of Legionella in low temperature district heating

It is possible to avoid Legionella in domestic hot water from low-temperature district heating using appropriate sterilization methods or improved technologies, and the solutions are low-cost and energy efficient.

These are the main results from 4DH PhD Fellow Xiaochen Yang who defended her PhD dissertation last Friday at DTU, Technical University of Denmark.

 - From the results of my project, low temperature district heating can be applied without risk of Legionella, without compromising energy efficiency, or increasing the operation cost. With new technologies or improved technologies in the substation, the low temperature district heating supply can be both energy-efficient and cost-efficient, said Xiaochen Yang.

She has investigated several solutions in low temperature district heating that meet comfort and hygiene requirements for domestic hot water preparation, which includes both model works and case studies. One case study is a multi-storey building with flat station system in Viborg, Denmark. The other is in Bjerringbro, Denmark, where 23 houses are supplied by ultra-low temperature district heating at 46°C, and she picked up 5 case houses for analysis.

Optimal solutions with good energy performance

- The optimal solutions can be different to specific situations. With low-temperature district heating, the flat station can be the best solution for the low-energy buildings in the future due to its good energy performance and low energy cost, while for the ultra-low temperature scenario, the direct electric heater or micro tank solution can be a good solution, explained Xiaochen Yang.

The flat station solution prevent Legionella by minimizing the total volume of domestic hot water, while the electric heater or micro tank solution use the theory of temperature boosting.

- For the case study in Viborg, since they are not using low-temperature district heating, we built a model to simulate the performance if low temperature district heating is applied. From the model results, the flat station system is able to reduce 30% energy for supplying domestic hot water inside the building compared to system with circulation. For the case study in Bjerringbro, the in-line electric heater system consumes less energy compared to other domestic hot water configurations since it has much less heat loss.

Valuable for DH companies and consumers

Xiaochen Yang expects that the recommendation of solutions to a specific case can be helpful information for district heating companies when planning an area with low temperature district heating supply. And the proposals of developing some of the existing technologies such as a smart control method for the electric heat tracing system, a flat station system with improvement on the bypass, and a micro tank system as an improvement for the direct electric heater system, could be very valuable to investigate by the industry, and realize in manufacture.

- Low temperature district heating is an efficient way of supplying heat to consumers. But since the security of domestic hot water supply is crucial for the heat users, we have to make sure the risk of Legionella is eliminated before we implement low temperature district heating.

Xiaochen Yang is now looking for research opportunities in the field of district heating.

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