Presentation of a distributed testing infrastructure for joint experiments across multiple remote laboratories for robust development of new district heating concepts

4th-generation district heating networks confront numerous challenges such as integrating decentralized renewable energy sources, bidirectional heat transfer, new storage concepts, low-temperature operation, custom heat supply, data management, and advanced control strategies. Laboratory and hardware-in-the-loop testing offer a safe, cost-effective environment for testing and validating these innovations. This paper presents a framework for joint experiments in multiple remote laboratories, enhancing the testing of district heating system components. This distributed testbed enhances the efficiency of testing by utilizing existing equipment and expertise from various laboratories, thereby reducing costs and time and allowing for more scenarios to test. It targets manufacturers, grid operators, and research institutions, facilitating collaborative lab work for technology testing before field deployment. This approach allows for diverse test scenarios, considering component interactions across different locations without identical hardware or software. The framework's efficacy is shown in a proof-of-concept with a low-temperature district heating network integrated across four Fraunhofer Institutes. An initial experiment connects a test building and a ground-source heat pump physically existing in different labs with emulated models of a district heating network and a geothermal source. Results from a three-week operation validate the framework's performance.