# Designing a Conceptual Digital Twin Architecture for High-Temperature Heat Upgrade Systems
## 4 High-Temperature Heat Upgrade Use Case  
### Physical System Perspective  
The physical system of the digital twin is represented by the high-temperature heat upgrade system prototype developed as part of the ongoing SUSHEAT project [9,10]. This innovative system will collect renewable energy, combined with the waste energy to be stored so it can be used at a later time in energy-intensive industrial processes. These types of systems are complex in nature and require process optimization and orchestration due to the high level of interdependencies between system components and parameters. A simple concept of such high high-temperature heat upgrade system is detailed in Figure 3, and it has the following components:  
* Waste heat--represents a low-temperature energy source. SUSHEAT targets industrial processes that generate residual heat that is usually lost in the environment.
* Low and High T Thermal Energy Storage--these are two Thermal Energy Storage (TES) devices for the two loops of the system: the cold loop and the hot loop. Useful in situations where there is no high heat demand and the waste energy can be stored in TESes, or when there is no waste heat available, the energy can be extracted from TESes. Two approaches are currently used to find a suitable TES design for the SUSHEAT prototype. One direction explores the use of biomimicry inspiration optimized by genetic algorithms [40] and built using additive manufacturing [41], while the other direction is to use a U-net CNN-based network [42]. The Low T TES is designed for 15 kWh, while the High T TES is aimed for 30 kWh.
* Heat exchangers--two heat exchangers, one for capturing the waste heat into the system, and one to transfer the high-temperature heat to the heat demand process.
* Heat pump--a custom-made heat pump that can output high temperature. Currently, a prototype of a Stirling-cycle heat pump designed for normal operation at 200 kWh with a maximum heat capacity of 400 kW is being tested in a biogas facility, and is able to provide heat up to 200 \({}^{\circ}\)C using a source as low as 10 \({}^{\circ}\)C with high efficiency, having a COP of up to 3 [43,44]. The SUSHEAT heat pump prototype aims at temperatures up to 250 \({}^{\circ}\)C to meet the demands of multiple industries.
* HTL tank buffers--two heat transfer liquid (HTL) buffers, one for each system loop. They are necessary for system safety and pressure damping, ensuring full system flexibility with different types of HTLs across different temperature ranges.
* Solar energy collectors--provide additional energy into the system. Depending on the heat demand, medium or high-temperature collectors can be used. In our case, Fresnel collectors, a type of high-temperature collector, will be used.
* Heat demand--this represents an industrial process that requires a high temperature to operate, usually between 150 \({}^{\circ}\)C and 250 \({}^{\circ}\)C, considering the current heat pump capabilities and market needs [11].  
Figure 3: The concept of a high-temperature heat upgrade system (adapted from [9,45]).  
To be able to test the SUSHEAT concept, we will use a simplified system, a laboratory rig, where we simulate the waste heat and Fresnel collectors using electric heaters, and the heat demand with a heat sink. The testing framework uses water as HTL. More details regarding the engineering considerations and preliminary design of the SUSHEAT laboratory rig can be found in [46].