IEA DHC Annex XIII Project 07: CASCADE - A comprehensive toolbox for integrating low-temperature sub-networks in existing district heating networks
Low temperature district heating networks (LTDHN) operating at supply temperatures of 50-70°C can very effectively utilize renewable heat sources such as solar or geothermal energy, ambient heat or low temperature waste heat.
However, the vast majority of urban district heating networks (DHN) operate on relatively high temperatures, typically 90-110°C supply and 50-70°C return. The high-temperature mode of operation has a certain robustness and high temperature DHNs cannot be easily transformed into a LTDHN, for example due to the characteristics of the building stock, the requirement of transporting sufficient amounts of energy with the given capacities and high temperature heat e.g. from waste incineration.
CASCADE investigates an alternative approach: the integration of LTDHNs into the return line of an existing large urban DHN, thus creating a sub-LTDHN. This will reduce the return temperature of the overall DHN and thus improve its efficiency and sustainability as well as enable increasing its capacity for connecting new customers. Consequently, sub-LTDHN can be a key enabler for the decarbonization of urban DHN by enabling an efficient utilization of local energy sources and have the potential to reduce substantially the overall network temperatures.
At the same time, a high temperature backup from the main network to the sub-LTDHN is available. Therefore, LTDHNs as sub-networks in the return line may represent a win-win situation.
Implementation of energy cascades by means of a sub-LTDHN is possible when (i) there is a well-established high-temperature DHN and there is a planned (greenfield) urban area or larger customers with the ability to utilize heat at low temperatures.
These conditions are typical for many cities planning new districts. The utilization of renewable and waste heat and storage depends on the local potentials and a coordination with the overall network management and supply portfolio development is pivotal, including the business models.
The project addresses the following questions: How does the sub-LTDHN change the economics and hydraulic behavior of the entire network? Especially when the subsystem uses local storage and low-temperature sources? Under what circumstances is a sub-LTDHN advantageous for the overall efficiency of the overall DHN or the existing plants and actually allows for a win-win-situation?
What is the best operation strategy of the sub-LTDHN (optimizing for main DHN friendly behavior or for local supply), in particular during peak demand? What is the economic value of the sub-LTDHN and does it compensate for the implicit additional costs? How to integrate sub-LTDHN without disrupting contractual obligations with other users? How many sub-LTDHN are possible in an urban DHN?
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