Efficient development of HVAC systems for electric commercial vehicles through a Hardware-in-the-Loop approach on the ThermoLab testbed

Efficient development of HVAC systems for electric commercial vehicles through a Hardware-in-the-Loop approach on the ThermoLab testbed | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Efficient development of HVAC systems for electric commercial vehicles through a Hardware-in-the-Loop approach on the ThermoLab testbed Luis Vincent Fiore, Christian Beidl, Erik Stenger, Niko Weimer, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8583608/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract The shift towards sustainable mobility in the passenger- and transport sectors is driving the development of sustainable propulsion technologies. Alongside the advancement of efficient and sustainable powertrains, research increasingly targets the optimization of development processes to address rising system complexity and reduce engineering effort. With the growing adoption of Battery Electric Vehicles (BEVs) and Fuel Cell Electric Vehicles (FCEVs), thermal management is becoming a critical part of the overall vehicle energy management. Considering electric commercial vehicles for public transport, one of the main challenges – beyond the optimal thermal conditioning of powertrain components – is the efficient thermal management of the passengers’ cabin, which significantly impacts overall energy consumption, vehicle range and thermal comfort especially at cold ambient conditions. Particular attention must be paid to the interaction between the HVAC system and the thermal management system of the powertrain components, as can be found in state-of-the-art heat pump systems for electrified vehicles. To enable the early development of operating strategies while accounting for system interactions and determining the optimal system layout, an integrated approach is applied that combines simulation with testing on the Hardware-in-the-Loop (HiL)-ThermoLab testbed. This approach is used in the EU Horizon Europe research project MINDED improving the energy consumption of a battery-electric IVECO eDaily minibus at 0°C ambient temperature. For the development of optimal HVAC layouts and efficient operating strategies, a multi-physical 1D digital twin model of the passenger cabin, described with the modeling language Modelica and validated with measurement results, is used, while all key refrigerant- and thermal circuit components are physically implemented on the testbed. Hardware components that are not yet available are substituted on the HiL-ThermoLab testbed using novel dynamic thermo-hydraulic emulators. The results of this study indicate that the combined use of simulation and hardware testing on the HiL-ThermoLab testbed offers substantial potential for reducing engineering effort in the development process of advanced energy management systems. Energy management Thermal Management Co-simulation HVAC-optimization Hardware-in-the-Loop testbed Cabin simulation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1 TECHNICAL BACKGROUND AND CHALLENGES The trend towards developing sustainable powertrain systems is leading to increased electrification of vehicles in the individual and commercial transport sectors. A key challenge here is the application of an optimal thermal management system (TMS). This is an integral part of the energy management and is responsible for regulating heat flows. The electric powertrain is highly complex in terms of the thermal limits of its components. Their efficiency, durability and availability are highly sensitive to temperature [ 1 , 2 ]. Significant challenges in development also arise with regard to the development of heating, ventilation, and air conditioning (HVAC) systems. Passenger comfort is directly related to electric range. Due to the low waste heat generated by the electric powertrain components, most of the energy required to heat up the cabin must be drawn from the battery. Depending on the selected driving scenario, even in passenger cars, the energy required to maintain cabin comfort at low ambient temperatures can exceed the energy required for propulsion, resulting in 50 % les driving range [ 3 , 4 ]. These factors are even more pronounced in commercial vehicles, such as electric buses. To ensure cabin comfort despite a large cabin volume with large glass surfaces and frequent door openings, electric heating elements are used in many applications. The energy required for these heaters is drawn directly from the battery. To solve this issue and to minimize the impact of the heating system on the vehicle range, other applications use additional fuel-powered heaters [ 5 , 2 ]. The use of heat pump systems can provide a decisive advantage here. Such refrigerant-based circuits utilize the physical effects of the transition of liquids into a gaseous phase and vice versa. This allows the environment to be used as a heat source even at low temperatures. Connecting the refrigerant circuit to the powertrain coolant circuits opens additional possibilities for utilizing residual heat from the powertrain components. In order to investigate the high system complexity and the interactions between the thermal system for the powertrain components, the HVAC system, and thermal cabin comfort, an integrated development approach must be used. This development process involves linking simulation and testing on a hardware-in-the-loop (HiL) thermal test bed (ThermoLab), enabling the potential of technical solutions to be assessed at an early stage in terms of efficiency, comfort, and safety-related aspects [ 6 , 7 ]. 2 HOLISTIC DEVELOPMENT METHODOLOGY FOR THERMAL MANAGEMENT SYSTEMS 2 HOLISTIC DEVELOPMENT METHODOLOGY FOR THERMAL MANAGEMENT SYSTEMS The ThermoLab methodology, shown in Fig. 1 , has been established as a comprehensive framework to support the hardware and software development of TMS throughout all phases of the development process [ 8 , 9 ]. On the hardware side, it enables component-level investigations, whereby components can be experimentally characterized, simulation input data can be generated, and numerical models can be validated [ 9 ]. At system level, the methodology is applied to examine thermal circuit layouts as well as HVAC layouts with regard to efficiency and functionality, and to verify the integration and performance of individual components within the overall system [ 1 ]. For the thermally representative transfer of boundary conditions provided by components that are not installed on the system testbed, thermal emulators developed specifically for dynamic coolant emulation (Dynamic Module III)[ 10 ] and conditionable air stream emulation (CASE) are used. Through the linkage of simulation and test on the HiL thermal system testbed, this methodical approach allows the calibration of controllers, the development and testing of operation strategies at an early stage of the development process in order to minimize engineering effort. The transfer to vehicle level on a 4x4 chassis dynamometer with low-temperature extension is then used to validate and calibrate the designed operating strategies and control algorithms [ 5 ]. This paper focuses on the implementation and assessment of the potential of a heat pump application in an electric minibus. For this purpose, a simplified refrigerant- and coolant circuit is set up on the thermal system testbed and linked to a multi-physical digital twin of the vehicles passenger cabin. 2.1 Emulation hardware at ThermoLab thermal system testbed The realistic transfer of boundary conditions to the testbed is provided by thermally representative simulation models. For this purpose, two different dynamic emulators were developed specifically for use at ThermoLab testbed. These differ in the medium to be conditioned, while the Dynamic Module III represents a highly dynamic conditioning unit for coolants [ 10 ], the CASE is used for the dynamic conditioning of air flows, for example for the flow through heat exchangers. The thermal behavior of components within a thermal circuit can be characterized by their physical properties and fluid dynamic properties, namely thermal mass, heat transfer coefficients, and internal heat sources. For thermal emulation, two aspects are considered: the representation of waste heat generation and the emulation of the component’s thermal mass acting as a heat sink. Depending on the specific test requirements and the available hardware, either the heat source, the heat sink, or both may be emulated, as shown in Fig. 2 . To achieve this, a co-simulation approach is required, as the thermal power – whether positive or negative – must be computed under boundary conditions provided by the testbed. Real hardware provides the most accurate representation of in-vehicle thermal behavior but requires both hardware availability and testbed compatibility and results in increased hardware complexity on the testbed. If integration is limited (e.g., safety constraints for high-voltage batteries), thermal losses can be emulated via heat sources, while the thermal mass can be approximated using substitute hardware with similar properties. The most flexible solution is the combined emulation of heat sources and sinks, enabling modular and scalable testing. The basic software approach of the two emulators used are identical in terms of their basic functional structure and are shown in Fig. 2 . The Dynamic Module III requires information about the heat flow to be transferred to the coolant in order to emulate the thermal behavior. This heat flow can be transferred from a thermally representative simulation model or specified as numeric value directly. In principle, it is therefore possible to control either a specific coolant outlet temperature to the unit under test (UUT) \(\:{\text{T}}_{\text{U}\text{U}\text{T}}\) of the emulator, which together with the existing volume flow results in a transferred heat flow \(\:{\dot{\text{Q}}}_{\text{c}\text{o}\text{o}\text{l}\text{a}\text{n}\text{t}}\) or control directly the heat flow \(\:{\dot{\text{Q}}}_{\text{c}\text{o}\text{o}\text{l}\text{a}\text{n}\text{t}}\) to be transferred into the thermal circuit. Depending on the selected control mode, the emulator's actuators are controlled. The basic principle involves mixing of coolant flows through a four-way valve in distribution mode. The challenge lies in ensuring sufficiently high dynamics while maintaining high steady-state control quality [ 10 , 12 ]. In the research project of the present study, Dynamic Module III was implemented to represent the boundary conditions of the environment at an ambient temperature of 0°C in the refrigerant and cooling circuit of the heat pump circuit. For this purpose, the control concept for a specific outlet temperature applied in Dynamic Module III was used to represent a constant heat sink. To control an outlet temperature to the UUT \(\:{\text{T}}_{\text{U}\text{U}\text{T}}\) the mass balance at the mixing point shown in Fig. 3 (left) is necessary. Assuming that the specific heat capacity \(\:\text{c}\text{p}\) is constant follows: \(\:{\dot{\text{m}}}_{\text{h}\text{o}\text{t}}\text{*}{\text{T}}_{\text{h}\text{o}\text{t}}+{\dot{\text{m}}}_{\text{c}\text{o}\text{l}\text{d}}\text{*}{\text{T}}_{\text{c}\text{o}\text{l}\text{d}}+{\dot{\text{m}}}_{\text{c}\text{e}\text{n}\text{t}\text{e}\text{r}}\text{*}{\text{T}}_{\text{c}\text{e}\text{n}\text{t}\text{e}\text{r}}=\) \(\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:{\dot{\text{m}}}_{\text{U}\text{U}\text{T}}\text{*}{\text{T}}_{\text{U}\text{U}\text{T}}\) (1) Since either \(\:{\dot{m}}_{hot}\) or \(\:{\dot{m}}_{cold}\) is set to zero for a valve angle α range of 90 degrees, the following case distinction applies to the mixing ratio: for -45°≤α≤0° ( \(\:{\dot{\text{m}}}_{\text{c}\text{o}\text{l}\text{d}}\) = 0) \(\:{{\chi\:}}_{1}=\:\frac{\:{\text{T}}_{\text{U}\text{U}\text{T}}-{\text{T}}_{\text{h}\text{o}\text{t}}}{{\text{T}}_{\text{c}\text{e}\text{n}\text{t}\text{e}\text{r}}-{\text{T}}_{\text{h}\text{o}\text{t}}}\) ,and for 0°≤α≤45° ( \(\:{\dot{\text{m}}}_{\text{h}\text{o}\text{t}}\) = 0) \(\:{{\chi\:}}_{2}=\:\frac{\:{\text{T}}_{\text{U}\text{U}\text{T}}-{\text{T}}_{\text{c}\text{o}\text{l}\text{d}}}{{\text{T}}_{\text{c}\text{e}\text{n}\text{t}\text{e}\text{r}}-{\text{T}}_{\text{c}\text{o}\text{l}\text{d}}}\) (2) The control architecture in the modules function software comprises a feedforward strategy that converts the desired component temperature \(\:{\text{T}}_{\text{U}\text{U}\text{T},\text{d}\text{e}\text{s}}\) into the corresponding mixing ratio. Based on the temperature levels, a valve angle is determined using a map-based approach. The remaining control error between \(\:{\text{T}}_{\text{U}\text{U}\text{T},\text{d}\text{e}\text{s}}\) and the actual component \(\:{\text{T}}_{\text{U}\text{U}\text{T}}\) temperature is compensated by an adaptive PID controller. This controller accounts for the coolant viscosity as a function of the temperature difference across the module and continuously adapts its control parameters to meet the requirements of highly dynamic medium conditioning while maintaining high steady-state accuracy. Detailed information on control logic is described in [ 10 ]. In this research paper, the CASE is used for the thermally representative transfer of the vehicle cabin to the ThermoLab testbed. The original interior heat exchanger of the electric minibus is used as UUT. The basic functionality consists of supplying or removing heat flows through the controlled conditioning of air mass flows. To supply a conditioned air mass flow, the CASE employs a heat exchanger integrated into the testbed infrastructure. Depending on the test requirements, it can be coupled to either a heat sink or -source, thereby enabling the provision of an air stream conditioned to a defined target temperature. The heat transfer to or from the heat exchanger used as the UUT can be controlled by adjusting the required air mass flow at a specified target temperature. However, operation at the exact target temperature is not mandatory and may entail disadvantages with respect to control dynamics, since any change in the air-side temperature requires a corresponding adjustment of the testbed heat exchanger temperature. To ensure consistent boundary conditions in the fluid circuit, the air mass flow through the UUT is regulated at a fixed air temperature \(\:{\text{T}}_{\text{a}\text{i}\text{r}}\) . The required heat flow is achieved by increasing or decreasing the air mass flow, while the fundamental thermodynamic equation is described by the heat balance of the conditioned heat exchanger, defined on the coolant side: $$\:{\dot{\text{Q}}}_{\text{U}\text{U}\text{T}}=\:\dot{\text{V}}\text{*}{\rho\:}\text{*}\text{c}\text{p}\text{*}\left({\text{T}}_{\text{i}\text{n}\text{U}\text{U}\text{T}}-{\text{T}}_{\text{o}\text{u}\text{t}\text{U}\text{U}\text{T}}\right)$$ 3 The control concept is based on regulating the heat flow on the coolant side. The controller is implemented as a PID controller with a map-based feedforward control, taking into account the transport delay on coolant side through the heat exchanger. The position of the dynamic slider acts as control variable for the regulation of airflow through the heat exchanger. 2.2 1D-MULTIPHYSICAL DIGITAL TWIN OF THE CABIN The 1D-digital twin of the cabin is modeled in Dymola using the object-oriented modeling language Modelica. The simulation model represents the thermal behavior of the cabin including the HVAC system interfaces. The simulation is parameterized using performance data of the simulated components. The vehicle’s control logics and the thermal behavior of the cabin is calibrated using results from initial vehicle measurements on a climatized chassis dyno [ 13 ]. For the integration into the ThermoLab HiL environment, the coolant circuit of the cabin heater was adapted by replacing the present PTC heater with boundary conditions imposed by the testbed. Depending on the coolant temperatures achieved with the heat pump configuration on the testbed, the thermal power transferred through the cabin coolant heat exchanger into the cabin, as well as the overall cabin warm-up behavior, is calculated. To ensure suitable real-time capability, the model was converted into a functional mockup unit. Within the model, a variety of temperatures in different areas of the cabin are calculated in order to achieve a detailed representation of the warm-up behavior. The following Fig. 4 shows the locations of the temperature sensors used for the simplified evaluation of the cabin heating in this study. 3 TESTBED SETUP FOR CO-SIMULATION AND PRELIMINARY INVESTIGATIONS With the integration of the thermal emulators (Dynamic Module III & CASE) and the 1D-digital twin of the vehicle’s passenger cabin into the co-simulation environment, the testbed setup is completed. The baseline vehicle is equipped with a thermal management system without a heat pump application. Cabin cooling for driver and passengers is realized via two separate refrigerant circuits, with the driver’s circuit additionally serving for indirect battery cooling through a chiller. Cabin and battery heating are provided electrically via PTC heaters, each with a nominal capacity of 5 kW for the passenger compartment, the driver, and the battery. In line with the adapted simulation model, the driver heating circuit is replaced by a heat pump configuration on the testbed utilizing the series-installed electric AC scroll compressor. The remaining heating elements for the battery and passenger cabin are represented in the simulation to evaluate cabin heating dynamics. This approach enables the reproduction of simulation-derived boundary conditions on the testbed and provides the basis to evaluate the potential of heat pump configuration under realistic boundary conditions. The Dynamic Module III shown in Fig. 5 represents the ambient conditions, which are specified by the simulation model at a constant 0°C ambient temperature for the purpose of these experiments. To this end, the Dynamic Module III controls a fluid temperature of 0°C at the evaporator of the refrigerant circuit, thereby serving as the ambient-representing heat sink for the system. In addition, a three-way valve (Precon valve) is integrated, which is solely used to precondition the cabin heater circuit to an initial temperature of 0°C at the beginning of each test. The driver cabin heater coolant circuit is fully implemented on the testbed, with the sole exception that the electric heating element is replaced by the condenser of the refrigerant circuit. The cabin heat exchanger is integrated into the CASE. Within this circuit, the volumetric flow rate demanded by the simulation model is regulated by an electric coolant pump. At the same time, the inlet temperature of the cabin coolant heat exchanger is fed back into the simulation model. In the simulation model, the heat transferred through the cabin heat exchanger into the vehicle is calculated based on this inlet temperature and the prevailing volumetric flow rate from the testbed. The calculated heating power transferred to the cabin is subsequently fed back to the testbed. This thermal power serves as a setpoint for the CASE, which continuously dissipates the exact amount of heat in order to ensure identical boundary conditions in the cabin heater coolant circuit. The following Fig. 6 shows an example of a sine wave curve used as setpoint ( \(\:{\dot{\text{Q}}}_{\text{U}\text{U}\text{T},\text{d}\text{e}\text{m}}\) ) for the CASE system. The dynamic control of the air flow using the dynamic slider allows the demanded heat flow to be dissipated via the heat exchanger with a high degree of accuracy ( \(\:{\dot{\text{Q}}}_{\text{U}\text{U}\text{T}}\) ). The average standard deviation from the demanded heat flow remains below 90 W for a wide range of operating scenarios. The functionality of the CASE system and its functional software could thus be validated. Figure 7 shows the testbed setup on the ThermoLab thermal system testbed used for all investigations in this research. The refrigerant circuit is marked with red arrows, showing the flow direction. The refrigerant circuit contains an oil absorber, which allowed using a Coriolis mass flowmeter for calculating the coefficient of performance (COP) in preliminary investigations below. Furthermore, an accumulator was integrated on the suction side to avoid liquid refrigerant reaching the compressor inlet. The evaporator is represented by a plate heat exchanger and is coupled with the Dynamic Module III for environment representation. As condenser a similar plate heat exchanger is used, which is coupled to the cabin heater coolant circuit. 3.1 Preliminary investigations To proof the function of the testbed setup, first investigations take place without the linkage to the simulation model. The goal of these preliminary investigations is to investigate: Adjustment of an appropriate refrigerant charge for the refrigerant circuit Quantification of the maximum heat rejection capacity at varying pressure ratios Suitability of the series compressor with R134a for a heat pump configuration The refrigerant charge of the refrigerant circuit in Fig. 5 has a significant impact on the overall system performance. Undersized refrigerant charges result in reduced mass flow rates through the evaporator accompanied by elevated suction pressures, which limit the heat absorption from the surrounding. In contrast, oversized refrigerant charges cause a pronounced increase in discharge pressure due to the accumulation of excess refrigerant in the condenser. In both cases, the consequence is a significantly reduced (COP) [ 14 , 15 ]. In the present study, the refrigerant charge was estimated based on the volume of the refrigerant circuit. The approach used is described in [ 16 ]. Based on this approximation, three different charge levels were selected and experimentally investigated. A refrigerant charge corresponding to an initial pressure of 5.8 bar(a) exhibited clear signs of overcharging, and operation outside the wet vapor area was only possible in a limited range of operating conditions. In contrast, charge levels of 4.5 bar(a) and 5.2 bar(a) enabled stable operation of the refrigerant circuit. The highest heating capacity and coefficient of performance for heating (COPh)​ were achieved at 5.2 bar(a). The specified initial pressures refer to the stationary state at 293.15 K refrigerant and ambient temperature. Experimental investigations reported in [ 14 ] and [ 17 ] have shown that the optimal refrigerant charge is always dependent on the boundary conditions and, therefore, cannot be determined exactly. However, the achievable heating and cooling capacities remain nearly constant over a wide range of under- or overcharging. Based on these findings, an initial charge pressure of 5.2 bar(a) is adopted for the subsequent experiments. For the quantification of the maximum achievable heat rejection using the testbed setup shown in Fig. 7 , the system was tested at constant boundary conditions of 0°C for various compressor speeds and pressure ratios. Therefore, the Dynamic Module III was used to ensure constant coolant temperatures on the coolant inlet of the evaporator and condenser throughout the stationary tests. Figure 8 shows the results of the test, the results are plotted in a map showing the rejected heat at the condenser for various compressor speeds and electronic expansion valve (EEXV) openings. In principle, the achievable heat output increases with compressor speed. However, it is also apparent that the position of the EEXV significantly influences the heat output. By varying the EEXV opening, the evaporator capacity is adjusted. A smaller valve opening results in a reduced refrigerant mass flow and simultaneously higher superheat. Consequently, the evaporator operates less efficiently, as a larger portion of the heat exchanger surface is no longer wetted by liquid refrigerant, leading to a reduction in the overall heat output. In addition, the isolines of equal COP for heating (COPh) lines are plotted. At peak performance, this test setup can dissipate a heat output of 4.7 kW under the given boundary conditions. This corresponds to the output of the PTC heating element used in the series vehicle, but the electrical energy requirement is lower for the heat pump system in accordance with the COPh. The achievable heat outputs are therefore of a comparable magnitude, and the series compressor shows potential for an initial assessment. In operating conditions where the thermal demand for cabin heating is reduced, a significant potential for lowering the compressor speed can be identified, which shows potential to further improve the COPh. 4 THERMOLAB IN THE LOOP – RESULTS OF A HEAT PUMP APPLICATION FOR AN ELECTRIC MINIBUS Following the completion of the preliminary investigations regarding the suitability of the compressor and the determination of the optimal refrigerant charge, the testbed setup was coupled with the co-simulation environment to evaluate cabin heating behavior based on the 1D-cabin model. As a test scenario, the cabin warm-up behavior at an ambient temperature of 0°C was selected. The applied driving cycle corresponds to five consecutive repetitions of the Worldwide harmonized light-duty vehicles test cycle (WLTC). Considering the specific characteristics of the electric minibus with respect to maximum speed and power-to-weight ratio, the WLTC Class 2 was chosen for the investigations and subsequently adapted to the vehicle’s maximum speed of < 85 km/h, while maintaining the driving distance analogous to the original WLTC Class 2 [ 18 ]. The resulting speed profile used can be seen in Fig. 9 . For the evaluation of the cabin comfort using the heat pump circuit on the testbed two operating cases are compared to the baseline results achieved using a PTC-heater for cabin heating. The boundary conditions of the refrigerant circuit were defined such that, by adjusting the pressure ratio via the EEXV, the evaporator temperature was maintained at 10 K below the ambient temperature of 0°C, enabling the extraction of heat from the environment. To ensure a rapid warm-up of the cabin heater coolant circuit at the beginning of each case, the compressor was operated at its maximum speed of 8000 rpm with the EEXV fully opened (100%) during the first 600 seconds. This operating strategy was directly derived from the preliminary characterization shown in Fig. 8 . During the measurements, for the heat pump operation (blue curve), the compressor was controlled to reach the maximum target temperature in the cabin heater coolant circuit. The resulting average condensing temperature is 52°C. For the second case an optimized heat pump operation (green curve) was targeted. The condensing temperature was reduced to 45°C in order to achieve a more efficient operation of the heat pump circuit. This adjustment allowed for lower compressor speeds and reduced pressure ratios, thereby improving the overall coefficient of performance (COPh) while maintaining adequate cabin heating for comfort. The corresponding average cabin temperature profiles over the measurement period are presented in Fig. 9 , illustrating the trade-off between heating rate and system efficiency. Considering the heat transferred through the heat exchanger for cabin heating, it becomes evident that both operating modes of the heat pump circuit exhibit a delayed warm-up compared to the baseline (“PTC”). This delay can be attributed to the increased thermal mass introduced by the integration of the condenser in the form of a plate heat exchanger relative to the PTC baseline configuration. Across the entire test duration, the heat output achieved with the heat pump operation remains below the one from the PTC heaters. This is partly due to the progressive warming of the cabin heater coolant circuit, which reduces the achievable heat transfer compared to the preliminary investigations shown in Fig. 8 . Additionally, the coolant circuit in the baseline vehicle with PTC operation reaches an approximately 10 K higher temperature level, which is limited by the operational constraints of the R134a refrigerant. This behavior can be explained by the isentropic compression process, considering the refrigerant properties and the compressor's isentropic efficiency [ 19 ]. This finding shows that, for a possible vehicle application, the cabin heat exchanger needs to be redesigned to achieve the same heat output at a lower entrance temperature difference. With regard to the average cabin temperature calculated from the measurement points shown in Fig. 4 , it becomes evident that the initial delay in heat delivery also results in a slightly reduced warm-up behavior. At the end of the test cycle, the cabin temperature for the standard heat pump operation is approximately 0.9 K lower than the baseline, and 1.7 K lower for the optimized heat pump operation. In terms of electrical energy consumption, the potential of a heat pump application becomes immediately apparent. Figure 10 presents the electrical energy consumption for heating the driver’s cabin, corresponding to the measurements shown in Fig. 9 . It should be noted that the passenger cabin and battery heating is included in the simulation but is not accounted for in this energy analysis. For electric heating via PTC, 6.30 kWh are required for the case depicted in Fig. 9 . The potential energy savings achieved with the heat pump system on the testbed can be seen in Fig. 10 . Operating at the maximum condenser temperature for the cabin heater coolant circuit reduces electrical energy consumption by around 22%. However, the same average cabin temperature is not reached, as shown in Fig. 9 . If this deficit is compensated using an additional PTC heater with an average efficiency of 96% [ 20 ], energy savings are still 7 % cmpared to the baseline PTC operation. Optimizing the heat pump circuit by reducing the condenser temperature to 45°C increases the potential energy savings to 18%. This improvement is primarily due to the lower discharge pressure associated with condenser temperature of 45°C, which also allows a reduction in compressor speed. Consequently, the specific work of the compressor decreases, and the volumetric efficiency of the compressor increases [ 20 ]. If a cabin temperature 1.7 K lower were to be accepted, energy savings of 43 % wuld be possible. Within the framework of the research project in which these investigations were conducted, a novel refrigerant compressor will be implemented in the future. Initial results indicate an efficiency increase of up to 20% in heat pump operation. Considering this improvement in the present analysis, it can be expected that energy consumption could be reduced by 30% while maintaining the same level of cabin comfort. 5 CONCLUSION Significant challenges in the development of thermal management systems, including HVAC architectures, need to be addressed at an early stage of the design process. The ThermoLab methodology, applied in this research project to evaluate the potential of a heat pump configuration, provides a framework to enable frontloading in system development. The approach combines the targeted use of thermal emulators and real hardware, embedded within a HiL environment, to facilitate system testing under realistic boundary conditions. With the Dynamic Module III and the CASE emulator, advanced thermal emulators are available, enabling the simulation-based transfer of boundary conditions to the ThermoLab thermal system testbed. For the conducted investigations, one of the three PTC heaters installed in the series vehicle was replaced by a simplified refrigerant circuit integrated into the ThermoLab testbed. The thermal boundary conditions during cabin heating were represented by a validated 1D-cabin model in the modelling language Modelica. The linkage of simulation and test with the simulation model was achieved through the HiL environment of the ThermoLab testbed, thereby enabling the representation of realistic transient boundary conditions. Preliminary investigations regarding the applicability of the series refrigerant compressor in a heat pump configuration demonstrated that its use is feasible for an initial evaluation of the concept’s potential. However, these investigations also highlighted limitations: neither the choice of refrigerant nor the configuration of the refrigerant circuit was optimized for achieving the required operating conditions. This indicates that the current setup is not sufficient for a future vehicle application, but it nevertheless serves as a valuable demonstration of a heat pump integration and its potential. The subsequent analysis of energy consumption revealed a significant improvement in efficiency when replacing the PTC heater with a heat pump configuration. This finding underlines the substantial potential of heat pump application for the electric minibus. The present study focused exclusively on substituting the cabin PTC heater, without addressing the broader integration of the heat pump into the overall HVAC architecture. Further potentials are expected for heating the passenger cabin and the high-voltage battery using a heat pump application. Within the framework of the EU Horizon Europe project MINDED, the development of a holistically optimized HVAC system is pursued. Central to this approach is the application of an innovative compressor specifically designed for heat pump operation, together with an optimized refrigerant circuit and control strategy. This concept is expected to significantly outperform the preliminary potential assessment presented here and to deliver a major contribution to the overall efficiency of electric minibuses, particularly at low ambient temperatures. Declarations Conflict of interest The authors declare no competing interest. Ethical approval Not applicable. Funding This study was funded by European Union’s Horizon research and innovation program under grant number 101138202. Author Contribution L.Fiore, C.Beidl, E.Stenger, N.Weimer, N.Kaiser, G.Hohenberg: Testbed setup, methodology, thermal emulators, literature research, test execution, test evaluation and test interpretation, review – editing. T. Bäuml, D. Dvorak: 1D-Multphysical digital twin of the cabin, review – editing. ACKNOWLEDGMENT This research was funded by European Union’s Horizon research and innovation program under grant number 101138202. The content of this publication is the sole responsibility of the authors and does not necessarily represent the view of the European Commission or its services. Data Availability No datasets were generated or analysed during the current study. References Fiore, L., Beidl, C., Stenger, E.: : Development of efficient thermal management systems for HEVs, BEVs and FCEVs using Co-simulation on the HiL-ThermoLab testbed. In :: Dritev 2024. VDI, S. 219–234. (2024) Pischinger, S., Genender, P., Klopstein, S., et al.: .: Aufgaben beim Thermomanagement von Hybrid- und Elektrofahrzeugen. In : ATZ - Automobiltechnische Z. 116 Heft 4, S. 54–59. (2014). https://doi.org/10.1007/s35148-014-0382-6 Wawzyniak, M., Wiebelt, A.: Thermomanagement für elektrifizierte Fahrzeuge. In : MTZ - Motortechnische Z. 77 Heft 5, S. 42–49. (2016). https://doi.org/10.1007/s35146-016-0030-7 Bareiß, M., Vorgerd, D.: Thermomanagement für elektrisch angetriebene Stadtbusse. In : ATZ - Automobiltechnische Z. 121 Heft 2, S. 52–55. (2019). https://doi.org/10.1007/s35148-018-0227-9 Peteranderl, C., Bernath, M., Tegethoff, W., et al.: .: Wärmepumpenmodule mit R744 für Stadtbusse. In : ATZ - Automobiltechnische Z. 124 2–3, S. 58–62. (2022). https://doi.org/10.1007/s35148-021-0806-z TP-103-13: Windshield Defrosting and Defogging Systems. LABORATORY TEST PROCEDURE, Ausgabe Juni (1996) Fiore, L., Conin, M., Beidl, C., et al.: Das ThermoLab als Entwicklungstool für innovative Thermalkreislaufsysteme. In: Heintzel, A. (Hrsg.): Experten-Forum Powertrain: Komponenten und Kompetenzen zukünftiger Antriebe 2022, Proceedings. Springer Fachmedien Wiesbaden, Wiesbaden, S. 26–39. (2023) Conin, M., Beidl, C., Kulzer, A.C.: : Konzeptionierung eines ganzheitlichen Thermomanagementsystems für ein Plug-in Hybrid Electric Vehicle (PHEV) Michael Conin. Technische Universität Darmstadt, Schriftenreihe des Instituts für Verbrennungskraftmaschinen und FahrzeugantriebeBand 30, Shaker, Düren, (2024) Heintzel, A.: (Hrsg.) : Experten-Forum Powertrain: Komponenten und Kompetenzen zukünftiger Antriebe 2022, Proceedings, Springer Fachmedien Wiesbaden, Wiesbaden, (2023) SAE NAPLES (: Hrsg.) : Simulation based dynamic thermal emulation unit for thermal preconditioning and emulation of hardware components on the thermal testbed. SAE Technical Paper Series, 2023-24-0096, Warrendale, PA, SAE International, SAE International, (2025) Conin, M., Fiore, L., Weimer, N.: : ThermoLab 2.0 – A Development Approach for Highly Efficient Thermal Management Systems for BEV and FCEV – In: 20 years of development methodology – 10 symposia Man and Methodology in the context of change: 10th International Symposium on Development Methodology 2023, Wiesbaden, 07–08 November 2023, Wiesbaden, 2023 Paulweber, M., Lebert, K.: Mess- und Prüfstandstechnik. Springer Fachmedien Wiesbaden, Wiesbaden, (2014) Marcus, S.Z.I.K.O.R.A., Marek, M.I.L.Č.E.V.I.Č., Lukas, A.C.K.E.R., Thomas, B.Ä.U.M.L.: Thermal and energy Management for INcreased Driving range of an Electric minibus including improved user- centric Design and thermal comfort – D5.1 Measurement results of initial vehicle identification, (2024). https://www.minded-project.eu/wp-content/uploads/2025/05/D1.5Measurement-results-of-initial-vehicle-identification.pdf [Zugriff am: 07.09.2025] Grace, I.N., Datta, D., Tassou, S.A.: : Sensitivity of refrigeration system performance to charge levels and parameters for on-line leak detection. In : Appl. Therm. Eng. 25 Heft 4, S. 557–566. (2005). https://doi.org/10.1016/j.applthermaleng.2004.07.008 Großmann, H.: : Pkw-Klimatisierung. Springer Berlin Heidelberg, Berlin, Heidelberg, (2013) Dipl: -Ing. Wolfgang Linck, Dipl. Ing. Manfred Giebe : Bestimmung von Kältemittelfüllmenge und Sammlergröße für Kälteanlagen – Wolfgang Linck, Friedberg und Manfred Giebe, Maintal. In :: Kälte und Klimatechnik, S. 64–70 Jan Christoph: Menken : Thermomanagement im batteriebetriebenen Pkw unter Nutzung eines Kaltdampfprozesses mit Sekundärkreislaufsystem – Von der Fakultät für Maschinenbau der Technischen Universität Carolo-Wilhelmina zu Braunschweig. Braunschweig, Universität Carolo-Wilhelmina zu Braunschweig, Dissertation, (2016) European Commission: COMMISSION REGULATION (EU): 2017/1151 – (EU) 2017/1151. European Commission (2017) Stephan, P., Schaber, K., Stephan, K.: : Thermodynamik. Springer Berlin Heidelberg, Berlin, Heidelberg, (2013) Park, M., Kim, S.: : Heating Performance Characteristics of High-Voltage PTC Heater for an Electric Vehicle. In : Energies 10 Heft 10, S. 1494. (2017). https://doi.org/10.3390/en10101494 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 18 Mar, 2026 Reviews received at journal 09 Mar, 2026 Reviews received at journal 17 Feb, 2026 Reviewers agreed at journal 09 Feb, 2026 Reviewers agreed at journal 06 Feb, 2026 Reviewers invited by journal 06 Feb, 2026 Editor assigned by journal 22 Jan, 2026 Submission checks completed at journal 15 Jan, 2026 First submitted to journal 12 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8583608","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":588540968,"identity":"32d820a8-46b9-4163-adf7-a0b52647cdcc","order_by":0,"name":"Luis Vincent Fiore","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBElEQVRIiWNgGAWjYBAC9mYQWcFgAOGyQYV5GBgYG3Bo4TkMIs+AtDDDtAAZPAl4tBwAEoxtJGlhZz724eO8w8a6DfzHJBjK7PJ1+88f/PD2B4NsPy4tzGzJM2duO2xmdoCZTYLhXLLlthvJzJJzEhiMZ+Kwxp6Zx5iZd1uaDUiL9N82ZgOzG8wM0kCHJW44gMsWoJa/cyBaJBjb6g3Mzh9m/g3Ssh+fFsYGGzOolsMGZgeS2SC24PELY88xG2Ozw8zGFgznjgMdlmxmOSdNwngGLlv4Dx9m+FEjYbjteOPDGwxl1UCHHXx8442NjWw/Du8jADMqV4KQ+lEwCkbBKBgFeAAAW9dPNgj0IC8AAAAASUVORK5CYII=","orcid":"","institution":"TU Darmstadt","correspondingAuthor":true,"prefix":"","firstName":"Luis","middleName":"Vincent","lastName":"Fiore","suffix":""},{"id":588540969,"identity":"6b9255b8-cf30-4cfb-8c5f-91a908471368","order_by":1,"name":"Christian Beidl","email":"","orcid":"","institution":"TU 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15:53:58","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8583608/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8583608/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102412514,"identity":"7d5d3db5-313d-4d0b-8e07-d43ee0629730","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":399012,"visible":true,"origin":"","legend":"\u003cp\u003eMethodical approach used at ThermoLab thermal system testbed and vehicle testbed [7, 11]\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/193a347d2fe7fdc8675053e2.png"},{"id":102412517,"identity":"c1cc2d75-b381-4861-a943-cbb078658085","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":318067,"visible":true,"origin":"","legend":"\u003cp\u003eApproaches for emulation of thermal behavior on thermal system testbed [1, 8]\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/5f453618ef9fe8bcab5a5e01.png"},{"id":102412515,"identity":"a4791c2c-febf-428b-847c-1f5ad866c281","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":129520,"visible":true,"origin":"","legend":"\u003cp\u003eThermal emulators to be used at ThermoLab testbed Dynamic Module III (left) and CASE (right)\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/02e33f47d988f36f011b6882.png"},{"id":102412516,"identity":"7ef994f1-b9cf-49f7-8cfd-d12a926d1b57","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":88293,"visible":true,"origin":"","legend":"\u003cp\u003eCabin model and temperature measurement for cabin comfort evaluation\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/e056759378a5ac9426e273f2.png"},{"id":102412522,"identity":"a3290a22-27d9-4f9c-beb5-1cd3f07bf865","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":111920,"visible":true,"origin":"","legend":"\u003cp\u003eArchitecture of ThermoLab testbed using co-simulation\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/5a41bda47d3c3c334eaa208c.png"},{"id":102412523,"identity":"3259a818-c82c-4635-9cc8-99308b430824","added_by":"auto","created_at":"2026-02-11 12:17:43","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":55182,"visible":true,"origin":"","legend":"\u003cp\u003eResults of dynamic air conditioning for cabin heat exchanger using CASE\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/95654e17ce0056b0f0e198f4.png"},{"id":102412520,"identity":"13dd6f65-6eb2-42b1-8c44-ddbceff5ea57","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":388880,"visible":true,"origin":"","legend":"\u003cp\u003eTestbed setup, red arrows indicate flow direction of refrigerant circuit, blue arrows indicate cabin heater coolant circuit\u003c/p\u003e","description":"","filename":"Figure7.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/ac202ee63e516704e4b6c153.png"},{"id":102745516,"identity":"1b271ccb-3aee-4a67-9201-d1100a1705f5","added_by":"auto","created_at":"2026-02-16 08:51:25","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":505381,"visible":true,"origin":"","legend":"\u003cp\u003ePreliminary investigation, stationary operation at various pressure ratios and compressor speeds with 0 °C coolant inlet temperature at condenser and evaporator\u003c/p\u003e","description":"","filename":"Figure8.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/ca8bfebb5ecb87f5858d6486.png"},{"id":102412518,"identity":"8a3384e8-d641-4b8f-887b-b63aa90106de","added_by":"auto","created_at":"2026-02-11 12:17:42","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":137336,"visible":true,"origin":"","legend":"\u003cp\u003eResults of the co-simulation of a heat pump circuit on the ThermoLab thermal system testbed\u003c/p\u003e","description":"","filename":"Figure9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/8abc1bd686d31c4ab42abd98.jpg"},{"id":102745515,"identity":"cb70a30b-49f6-4245-be3a-08393f2a8d04","added_by":"auto","created_at":"2026-02-16 08:51:24","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1173466,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy consumption for driver’s cabin heating using PTC heater (baseline) \u0026amp; heat pump system\u003c/p\u003e","description":"","filename":"Figure10.png","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/467907c1de62836fa7464bba.png"},{"id":107704495,"identity":"4fcce2c2-68e8-423d-af75-858d24836af7","added_by":"auto","created_at":"2026-04-24 08:45:41","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3558108,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8583608/v1/559b34cd-8343-41d5-ac85-e3c8be13923e.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Efficient development of HVAC systems for electric commercial vehicles through a Hardware-in-the-Loop approach on the ThermoLab testbed","fulltext":[{"header":"1 TECHNICAL BACKGROUND AND CHALLENGES","content":"\u003cp\u003eThe trend towards developing sustainable powertrain systems is leading to increased electrification of vehicles in the individual and commercial transport sectors. A key challenge here is the application of an optimal thermal management system (TMS). This is an integral part of the energy management and is responsible for regulating heat flows. The electric powertrain is highly complex in terms of the thermal limits of its components. Their efficiency, durability and availability are highly sensitive to temperature [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Significant challenges in development also arise with regard to the development of heating, ventilation, and air conditioning (HVAC) systems. Passenger comfort is directly related to electric range. Due to the low waste heat generated by the electric powertrain components, most of the energy required to heat up the cabin must be drawn from the battery. Depending on the selected driving scenario, even in passenger cars, the energy required to maintain cabin comfort at low ambient temperatures can exceed the energy required for propulsion, resulting in 50 % les driving range [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThese factors are even more pronounced in commercial vehicles, such as electric buses. To ensure cabin comfort despite a large cabin volume with large glass surfaces and frequent door openings, electric heating elements are used in many applications. The energy required for these heaters is drawn directly from the battery. To solve this issue and to minimize the impact of the heating system on the vehicle range, other applications use additional fuel-powered heaters\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe use of heat pump systems can provide a decisive advantage here. Such refrigerant-based circuits utilize the physical effects of the transition of liquids into a gaseous phase and vice versa. This allows the environment to be used as a heat source even at low temperatures. Connecting the refrigerant circuit to the powertrain coolant circuits opens additional possibilities for utilizing residual heat from the powertrain components. In order to investigate the high system complexity and the interactions between the thermal system for the powertrain components, the HVAC system, and thermal cabin comfort, an integrated development approach must be used. This development process involves linking simulation and testing on a hardware-in-the-loop (HiL) thermal test bed (ThermoLab), enabling the potential of technical solutions to be assessed at an early stage in terms of efficiency, comfort, and safety-related aspects [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e"},{"header":"2 HOLISTIC DEVELOPMENT METHODOLOGY FOR THERMAL MANAGEMENT SYSTEMS","content":"\u003cdiv class=\"Heading\"\u003e2 HOLISTIC DEVELOPMENT METHODOLOGY FOR THERMAL MANAGEMENT SYSTEMS\u003c/div\u003e \u003cp\u003eThe ThermoLab methodology, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, has been established as a comprehensive framework to support the hardware and software development of TMS throughout all phases of the development process [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. On the hardware side, it enables component-level investigations, whereby components can be experimentally characterized, simulation input data can be generated, and numerical models can be validated [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. At system level, the methodology is applied to examine thermal circuit layouts as well as HVAC layouts with regard to efficiency and functionality, and to verify the integration and performance of individual components within the overall system [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. For the thermally representative transfer of boundary conditions provided by components that are not installed on the system testbed, thermal emulators developed specifically for dynamic coolant emulation (Dynamic Module III)[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] and conditionable air stream emulation (CASE) are used. Through the linkage of simulation and test on the HiL thermal system testbed, this methodical approach allows the calibration of controllers, the development and testing of operation strategies at an early stage of the development process in order to minimize engineering effort. The transfer to vehicle level on a 4x4 chassis dynamometer with low-temperature extension is then used to validate and calibrate the designed operating strategies and control algorithms [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis paper focuses on the implementation and assessment of the potential of a heat pump application in an electric minibus. For this purpose, a simplified refrigerant- and coolant circuit is set up on the thermal system testbed and linked to a multi-physical digital twin of the vehicles passenger cabin.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Emulation hardware at ThermoLab thermal system testbed\u003c/h2\u003e \u003cp\u003eThe realistic transfer of boundary conditions to the testbed is provided by thermally representative simulation models. For this purpose, two different dynamic emulators were developed specifically for use at ThermoLab testbed. These differ in the medium to be conditioned, while the Dynamic Module III represents a highly dynamic conditioning unit for coolants [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], the CASE is used for the dynamic conditioning of air flows, for example for the flow through heat exchangers.\u003c/p\u003e \u003cp\u003eThe thermal behavior of components within a thermal circuit can be characterized by their physical properties and fluid dynamic properties, namely thermal mass, heat transfer coefficients, and internal heat sources. For thermal emulation, two aspects are considered: the representation of waste heat generation and the emulation of the component\u0026rsquo;s thermal mass acting as a heat sink. Depending on the specific test requirements and the available hardware, either the heat source, the heat sink, or both may be emulated, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. To achieve this, a co-simulation approach is required, as the thermal power \u0026ndash; whether positive or negative \u0026ndash; must be computed under boundary conditions provided by the testbed. Real hardware provides the most accurate representation of in-vehicle thermal behavior but requires both hardware availability and testbed compatibility and results in increased hardware complexity on the testbed. If integration is limited (e.g., safety constraints for high-voltage batteries), thermal losses can be emulated via heat sources, while the thermal mass can be approximated using substitute hardware with similar properties. The most flexible solution is the combined emulation of heat sources and sinks, enabling modular and scalable testing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe basic software approach of the two emulators used are identical in terms of their basic functional structure and are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The Dynamic Module III requires information about the heat flow to be transferred to the coolant in order to emulate the thermal behavior. This heat flow can be transferred from a thermally representative simulation model or specified as numeric value directly. In principle, it is therefore possible to control either a specific coolant outlet temperature to the unit under test (UUT) \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T}}\\)\u003c/span\u003e\u003c/span\u003e of the emulator, which together with the existing volume flow results in a transferred heat flow \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{Q}}}_{\\text{c}\\text{o}\\text{o}\\text{l}\\text{a}\\text{n}\\text{t}}\\)\u003c/span\u003e\u003c/span\u003e or control directly the heat flow \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{Q}}}_{\\text{c}\\text{o}\\text{o}\\text{l}\\text{a}\\text{n}\\text{t}}\\)\u003c/span\u003e\u003c/span\u003e to be transferred into the thermal circuit. Depending on the selected control mode, the emulator's actuators are controlled. The basic principle involves mixing of coolant flows through a four-way valve in distribution mode. The challenge lies in ensuring sufficiently high dynamics while maintaining high steady-state control quality [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn the research project of the present study, Dynamic Module III was implemented to represent the boundary conditions of the environment at an ambient temperature of 0\u0026deg;C in the refrigerant and cooling circuit of the heat pump circuit. For this purpose, the control concept for a specific outlet temperature applied in Dynamic Module III was used to represent a constant heat sink. To control an outlet temperature to the UUT \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T}}\\)\u003c/span\u003e\u003c/span\u003e the mass balance at the mixing point shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (left) is necessary. Assuming that the specific heat capacity \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{c}\\text{p}\\)\u003c/span\u003e\u003c/span\u003e is constant follows:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{m}}}_{\\text{h}\\text{o}\\text{t}}\\text{*}{\\text{T}}_{\\text{h}\\text{o}\\text{t}}+{\\dot{\\text{m}}}_{\\text{c}\\text{o}\\text{l}\\text{d}}\\text{*}{\\text{T}}_{\\text{c}\\text{o}\\text{l}\\text{d}}+{\\dot{\\text{m}}}_{\\text{c}\\text{e}\\text{n}\\text{t}\\text{e}\\text{r}}\\text{*}{\\text{T}}_{\\text{c}\\text{e}\\text{n}\\text{t}\\text{e}\\text{r}}=\\)\u003c/span\u003e \u003c/span\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:{\\dot{\\text{m}}}_{\\text{U}\\text{U}\\text{T}}\\text{*}{\\text{T}}_{\\text{U}\\text{U}\\text{T}}\\)\u003c/span\u003e \u003c/span\u003e(1)\u003c/p\u003e \u003cp\u003eSince either \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{m}}_{hot}\\)\u003c/span\u003e\u003c/span\u003e or \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{m}}_{cold}\\)\u003c/span\u003e\u003c/span\u003e is set to zero for a valve angle α range of 90 degrees, the following case distinction applies to the mixing ratio:\u003c/p\u003e \u003cp\u003efor -45\u0026deg;\u0026le;α\u0026le;0\u0026deg; (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{m}}}_{\\text{c}\\text{o}\\text{l}\\text{d}}\\)\u003c/span\u003e\u003c/span\u003e= 0) \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\chi\\:}}_{1}=\\:\\frac{\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T}}-{\\text{T}}_{\\text{h}\\text{o}\\text{t}}}{{\\text{T}}_{\\text{c}\\text{e}\\text{n}\\text{t}\\text{e}\\text{r}}-{\\text{T}}_{\\text{h}\\text{o}\\text{t}}}\\)\u003c/span\u003e\u003c/span\u003e ,and for 0\u0026deg;\u0026le;α\u0026le;45\u0026deg; (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{m}}}_{\\text{h}\\text{o}\\text{t}}\\)\u003c/span\u003e\u003c/span\u003e= 0) \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\chi\\:}}_{2}=\\:\\frac{\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T}}-{\\text{T}}_{\\text{c}\\text{o}\\text{l}\\text{d}}}{{\\text{T}}_{\\text{c}\\text{e}\\text{n}\\text{t}\\text{e}\\text{r}}-{\\text{T}}_{\\text{c}\\text{o}\\text{l}\\text{d}}}\\)\u003c/span\u003e\u003c/span\u003e(2)\u003c/p\u003e \u003cp\u003eThe control architecture in the modules function software comprises a feedforward strategy that converts the desired component temperature \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T},\\text{d}\\text{e}\\text{s}}\\)\u003c/span\u003e\u003c/span\u003e into the corresponding mixing ratio. Based on the temperature levels, a valve angle is determined using a map-based approach. The remaining control error between \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T},\\text{d}\\text{e}\\text{s}}\\)\u003c/span\u003e\u003c/span\u003e and the actual component \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{T}}_{\\text{U}\\text{U}\\text{T}}\\)\u003c/span\u003e\u003c/span\u003e temperature is compensated by an adaptive PID controller. This controller accounts for the coolant viscosity as a function of the temperature difference across the module and continuously adapts its control parameters to meet the requirements of highly dynamic medium conditioning while maintaining high steady-state accuracy. Detailed information on control logic is described in [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this research paper, the CASE is used for the thermally representative transfer of the vehicle cabin to the ThermoLab testbed. The original interior heat exchanger of the electric minibus is used as UUT. The basic functionality consists of supplying or removing heat flows through the controlled conditioning of air mass flows. To supply a conditioned air mass flow, the CASE employs a heat exchanger integrated into the testbed infrastructure. Depending on the test requirements, it can be coupled to either a heat sink or -source, thereby enabling the provision of an air stream conditioned to a defined target temperature. The heat transfer to or from the heat exchanger used as the UUT can be controlled by adjusting the required air mass flow at a specified target temperature. However, operation at the exact target temperature is not mandatory and may entail disadvantages with respect to control dynamics, since any change in the air-side temperature requires a corresponding adjustment of the testbed heat exchanger temperature. To ensure consistent boundary conditions in the fluid circuit, the air mass flow through the UUT is regulated at a fixed air temperature \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{T}}_{\\text{a}\\text{i}\\text{r}}\\)\u003c/span\u003e\u003c/span\u003e. The required heat flow is achieved by increasing or decreasing the air mass flow, while the fundamental thermodynamic equation is described by the heat balance of the conditioned heat exchanger, defined on the coolant side:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{\\dot{\\text{Q}}}_{\\text{U}\\text{U}\\text{T}}=\\:\\dot{\\text{V}}\\text{*}{\\rho\\:}\\text{*}\\text{c}\\text{p}\\text{*}\\left({\\text{T}}_{\\text{i}\\text{n}\\text{U}\\text{U}\\text{T}}-{\\text{T}}_{\\text{o}\\text{u}\\text{t}\\text{U}\\text{U}\\text{T}}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe control concept is based on regulating the heat flow on the coolant side. The controller is implemented as a PID controller with a map-based feedforward control, taking into account the transport delay on coolant side through the heat exchanger. The position of the dynamic slider acts as control variable for the regulation of airflow through the heat exchanger.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 1D-MULTIPHYSICAL DIGITAL TWIN OF THE CABIN\u003c/h2\u003e \u003cp\u003eThe 1D-digital twin of the cabin is modeled in Dymola using the object-oriented modeling language Modelica. The simulation model represents the thermal behavior of the cabin including the HVAC system interfaces. The simulation is parameterized using performance data of the simulated components. The vehicle\u0026rsquo;s control logics and the thermal behavior of the cabin is calibrated using results from initial vehicle measurements on a climatized chassis dyno [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFor the integration into the ThermoLab HiL environment, the coolant circuit of the cabin heater was adapted by replacing the present PTC heater with boundary conditions imposed by the testbed. Depending on the coolant temperatures achieved with the heat pump configuration on the testbed, the thermal power transferred through the cabin coolant heat exchanger into the cabin, as well as the overall cabin warm-up behavior, is calculated. To ensure suitable real-time capability, the model was converted into a functional mockup unit. Within the model, a variety of temperatures in different areas of the cabin are calculated in order to achieve a detailed representation of the warm-up behavior. The following Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the locations of the temperature sensors used for the simplified evaluation of the cabin heating in this study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3 TESTBED SETUP FOR CO-SIMULATION AND PRELIMINARY INVESTIGATIONS","content":"\u003cp\u003eWith the integration of the thermal emulators (Dynamic Module III \u0026amp; CASE) and the 1D-digital twin of the vehicle\u0026rsquo;s passenger cabin into the co-simulation environment, the testbed setup is completed. The baseline vehicle is equipped with a thermal management system without a heat pump application. Cabin cooling for driver and passengers is realized via two separate refrigerant circuits, with the driver\u0026rsquo;s circuit additionally serving for indirect battery cooling through a chiller. Cabin and battery heating are provided electrically via PTC heaters, each with a nominal capacity of 5 kW for the passenger compartment, the driver, and the battery. In line with the adapted simulation model, the driver heating circuit is replaced by a heat pump configuration on the testbed utilizing the series-installed electric AC scroll compressor. The remaining heating elements for the battery and passenger cabin are represented in the simulation to evaluate cabin heating dynamics. This approach enables the reproduction of simulation-derived boundary conditions on the testbed and provides the basis to evaluate the potential of heat pump configuration under realistic boundary conditions.\u003c/p\u003e \u003cp\u003eThe Dynamic Module III shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e represents the ambient conditions, which are specified by the simulation model at a constant 0\u0026deg;C ambient temperature for the purpose of these experiments. To this end, the Dynamic Module III controls a fluid temperature of 0\u0026deg;C at the evaporator of the refrigerant circuit, thereby serving as the ambient-representing heat sink for the system. In addition, a three-way valve (Precon valve) is integrated, which is solely used to precondition the cabin heater circuit to an initial temperature of 0\u0026deg;C at the beginning of each test. The driver cabin heater coolant circuit is fully implemented on the testbed, with the sole exception that the electric heating element is replaced by the condenser of the refrigerant circuit. The cabin heat exchanger is integrated into the CASE. Within this circuit, the volumetric flow rate demanded by the simulation model is regulated by an electric coolant pump. At the same time, the inlet temperature of the cabin coolant heat exchanger is fed back into the simulation model. In the simulation model, the heat transferred through the cabin heat exchanger into the vehicle is calculated based on this inlet temperature and the prevailing volumetric flow rate from the testbed. The calculated heating power transferred to the cabin is subsequently fed back to the testbed. This thermal power serves as a setpoint for the CASE, which continuously dissipates the exact amount of heat in order to ensure identical boundary conditions in the cabin heater coolant circuit.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe following Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows an example of a sine wave curve used as setpoint (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{Q}}}_{\\text{U}\\text{U}\\text{T},\\text{d}\\text{e}\\text{m}}\\)\u003c/span\u003e\u003c/span\u003e) for the CASE system. The dynamic control of the air flow using the dynamic slider allows the demanded heat flow to be dissipated via the heat exchanger with a high degree of accuracy (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\dot{\\text{Q}}}_{\\text{U}\\text{U}\\text{T}}\\)\u003c/span\u003e\u003c/span\u003e). The average standard deviation from the demanded heat flow remains below 90 W for a wide range of operating scenarios. The functionality of the CASE system and its functional software could thus be validated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the testbed setup on the ThermoLab thermal system testbed used for all investigations in this research. The refrigerant circuit is marked with red arrows, showing the flow direction. The refrigerant circuit contains an oil absorber, which allowed using a Coriolis mass flowmeter for calculating the coefficient of performance (COP) in preliminary investigations below. Furthermore, an accumulator was integrated on the suction side to avoid liquid refrigerant reaching the compressor inlet. The evaporator is represented by a plate heat exchanger and is coupled with the Dynamic Module III for environment representation. As condenser a similar plate heat exchanger is used, which is coupled to the cabin heater coolant circuit.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Preliminary investigations\u003c/h2\u003e \u003cp\u003eTo proof the function of the testbed setup, first investigations take place without the linkage to the simulation model. The goal of these preliminary investigations is to investigate:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eAdjustment of an appropriate refrigerant charge for the refrigerant circuit\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eQuantification of the maximum heat rejection capacity at varying pressure ratios\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eSuitability of the series compressor with R134a for a heat pump configuration\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThe refrigerant charge of the refrigerant circuit in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e has a significant impact on the overall system performance. Undersized refrigerant charges result in reduced mass flow rates through the evaporator accompanied by elevated suction pressures, which limit the heat absorption from the surrounding. In contrast, oversized refrigerant charges cause a pronounced increase in discharge pressure due to the accumulation of excess refrigerant in the condenser. In both cases, the consequence is a significantly reduced (COP) [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn the present study, the refrigerant charge was estimated based on the volume of the refrigerant circuit. The approach used is described in [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Based on this approximation, three different charge levels were selected and experimentally investigated. A refrigerant charge corresponding to an initial pressure of 5.8 bar(a) exhibited clear signs of overcharging, and operation outside the wet vapor area was only possible in a limited range of operating conditions. In contrast, charge levels of 4.5 bar(a) and 5.2 bar(a) enabled stable operation of the refrigerant circuit. The highest heating capacity and coefficient of performance for heating (COPh)​ were achieved at 5.2 bar(a). The specified initial pressures refer to the stationary state at 293.15 K refrigerant and ambient temperature. Experimental investigations reported in [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] and [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] have shown that the optimal refrigerant charge is always dependent on the boundary conditions and, therefore, cannot be determined exactly. However, the achievable heating and cooling capacities remain nearly constant over a wide range of under- or overcharging. Based on these findings, an initial charge pressure of 5.2 bar(a) is adopted for the subsequent experiments.\u003c/p\u003e \u003cp\u003eFor the quantification of the maximum achievable heat rejection using the testbed setup shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the system was tested at constant boundary conditions of 0\u0026deg;C for various compressor speeds and pressure ratios. Therefore, the Dynamic Module III was used to ensure constant coolant temperatures on the coolant inlet of the evaporator and condenser throughout the stationary tests. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the results of the test, the results are plotted in a map showing the rejected heat at the condenser for various compressor speeds and electronic expansion valve (EEXV) openings. In principle, the achievable heat output increases with compressor speed. However, it is also apparent that the position of the EEXV significantly influences the heat output. By varying the EEXV opening, the evaporator capacity is adjusted. A smaller valve opening results in a reduced refrigerant mass flow and simultaneously higher superheat. Consequently, the evaporator operates less efficiently, as a larger portion of the heat exchanger surface is no longer wetted by liquid refrigerant, leading to a reduction in the overall heat output. In addition, the isolines of equal COP for heating (COPh) lines are plotted. At peak performance, this test setup can dissipate a heat output of 4.7 kW under the given boundary conditions. This corresponds to the output of the PTC heating element used in the series vehicle, but the electrical energy requirement is lower for the heat pump system in accordance with the COPh. The achievable heat outputs are therefore of a comparable magnitude, and the series compressor shows potential for an initial assessment. In operating conditions where the thermal demand for cabin heating is reduced, a significant potential for lowering the compressor speed can be identified, which shows potential to further improve the COPh.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 THERMOLAB IN THE LOOP – RESULTS OF A HEAT PUMP APPLICATION FOR AN ELECTRIC MINIBUS","content":"\u003cp\u003eFollowing the completion of the preliminary investigations regarding the suitability of the compressor and the determination of the optimal refrigerant charge, the testbed setup was coupled with the co-simulation environment to evaluate cabin heating behavior based on the 1D-cabin model. As a test scenario, the cabin warm-up behavior at an ambient temperature of 0\u0026deg;C was selected. The applied driving cycle corresponds to five consecutive repetitions of the Worldwide harmonized light-duty vehicles test cycle (WLTC). Considering the specific characteristics of the electric minibus with respect to maximum speed and power-to-weight ratio, the WLTC Class 2 was chosen for the investigations and subsequently adapted to the vehicle\u0026rsquo;s maximum speed of \u0026lt;\u0026thinsp;85 km/h, while maintaining the driving distance analogous to the original WLTC Class 2 [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The resulting speed profile used can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eFor the evaluation of the cabin comfort using the heat pump circuit on the testbed two operating cases are compared to the baseline results achieved using a PTC-heater for cabin heating.\u003c/p\u003e \u003cp\u003eThe boundary conditions of the refrigerant circuit were defined such that, by adjusting the pressure ratio via the EEXV, the evaporator temperature was maintained at 10 K below the ambient temperature of 0\u0026deg;C, enabling the extraction of heat from the environment.\u003c/p\u003e \u003cp\u003eTo ensure a rapid warm-up of the cabin heater coolant circuit at the beginning of each case, the compressor was operated at its maximum speed of 8000 rpm with the EEXV fully opened (100%) during the first 600 seconds. This operating strategy was directly derived from the preliminary characterization shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eDuring the measurements, for the heat pump operation (blue curve), the compressor was controlled to reach the maximum target temperature in the cabin heater coolant circuit. The resulting average condensing temperature is 52\u0026deg;C. For the second case an optimized heat pump operation (green curve) was targeted. The condensing temperature was reduced to 45\u0026deg;C in order to achieve a more efficient operation of the heat pump circuit. This adjustment allowed for lower compressor speeds and reduced pressure ratios, thereby improving the overall coefficient of performance (COPh) while maintaining adequate cabin heating for comfort. The corresponding average cabin temperature profiles over the measurement period are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, illustrating the trade-off between heating rate and system efficiency.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConsidering the heat transferred through the heat exchanger for cabin heating, it becomes evident that both operating modes of the heat pump circuit exhibit a delayed warm-up compared to the baseline (\u0026ldquo;PTC\u0026rdquo;). This delay can be attributed to the increased thermal mass introduced by the integration of the condenser in the form of a plate heat exchanger relative to the PTC baseline configuration. Across the entire test duration, the heat output achieved with the heat pump operation remains below the one from the PTC heaters. This is partly due to the progressive warming of the cabin heater coolant circuit, which reduces the achievable heat transfer compared to the preliminary investigations shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. Additionally, the coolant circuit in the baseline vehicle with PTC operation reaches an approximately 10 K higher temperature level, which is limited by the operational constraints of the R134a refrigerant. This behavior can be explained by the isentropic compression process, considering the refrigerant properties and the compressor's isentropic efficiency [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThis finding shows that, for a possible vehicle application, the cabin heat exchanger needs to be redesigned to achieve the same heat output at a lower entrance temperature difference. With regard to the average cabin temperature calculated from the measurement points shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, it becomes evident that the initial delay in heat delivery also results in a slightly reduced warm-up behavior. At the end of the test cycle, the cabin temperature for the standard heat pump operation is approximately 0.9 K lower than the baseline, and 1.7 K lower for the optimized heat pump operation.\u003c/p\u003e \u003cp\u003eIn terms of electrical energy consumption, the potential of a heat pump application becomes immediately apparent. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e presents the electrical energy consumption for heating the driver\u0026rsquo;s cabin, corresponding to the measurements shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eIt should be noted that the passenger cabin and battery heating is included in the simulation but is not accounted for in this energy analysis. For electric heating via PTC, 6.30 kWh are required for the case depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe potential energy savings achieved with the heat pump system on the testbed can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Operating at the maximum condenser temperature for the cabin heater coolant circuit reduces electrical energy consumption by around 22%. However, the same average cabin temperature is not reached, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. If this deficit is compensated using an additional PTC heater with an average efficiency of 96% [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], energy savings are still 7 % cmpared to the baseline PTC operation.\u003c/p\u003e \u003cp\u003eOptimizing the heat pump circuit by reducing the condenser temperature to 45\u0026deg;C increases the potential energy savings to 18%. This improvement is primarily due to the lower discharge pressure associated with condenser temperature of 45\u0026deg;C, which also allows a reduction in compressor speed. Consequently, the specific work of the compressor decreases, and the volumetric efficiency of the compressor increases [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. If a cabin temperature 1.7 K lower were to be accepted, energy savings of 43 % wuld be possible.\u003c/p\u003e \u003cp\u003eWithin the framework of the research project in which these investigations were conducted, a novel refrigerant compressor will be implemented in the future. Initial results indicate an efficiency increase of up to 20% in heat pump operation. Considering this improvement in the present analysis, it can be expected that energy consumption could be reduced by 30% while maintaining the same level of cabin comfort.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5 CONCLUSION","content":"\u003cp\u003eSignificant challenges in the development of thermal management systems, including HVAC architectures, need to be addressed at an early stage of the design process. The ThermoLab methodology, applied in this research project to evaluate the potential of a heat pump configuration, provides a framework to enable frontloading in system development. The approach combines the targeted use of thermal emulators and real hardware, embedded within a HiL environment, to facilitate system testing under realistic boundary conditions. With the Dynamic Module III and the CASE emulator, advanced thermal emulators are available, enabling the simulation-based transfer of boundary conditions to the ThermoLab thermal system testbed.\u003c/p\u003e \u003cp\u003eFor the conducted investigations, one of the three PTC heaters installed in the series vehicle was replaced by a simplified refrigerant circuit integrated into the ThermoLab testbed. The thermal boundary conditions during cabin heating were represented by a validated 1D-cabin model in the modelling language Modelica. The linkage of simulation and test with the simulation model was achieved through the HiL environment of the ThermoLab testbed, thereby enabling the representation of realistic transient boundary conditions.\u003c/p\u003e \u003cp\u003ePreliminary investigations regarding the applicability of the series refrigerant compressor in a heat pump configuration demonstrated that its use is feasible for an initial evaluation of the concept\u0026rsquo;s potential. However, these investigations also highlighted limitations: neither the choice of refrigerant nor the configuration of the refrigerant circuit was optimized for achieving the required operating conditions. This indicates that the current setup is not sufficient for a future vehicle application, but it nevertheless serves as a valuable demonstration of a heat pump integration and its potential.\u003c/p\u003e \u003cp\u003eThe subsequent analysis of energy consumption revealed a significant improvement in efficiency when replacing the PTC heater with a heat pump configuration. This finding underlines the substantial potential of heat pump application for the electric minibus. The present study focused exclusively on substituting the cabin PTC heater, without addressing the broader integration of the heat pump into the overall HVAC architecture. Further potentials are expected for heating the passenger cabin and the high-voltage battery using a heat pump application.\u003c/p\u003e \u003cp\u003eWithin the framework of the EU Horizon Europe project MINDED, the development of a holistically optimized HVAC system is pursued. Central to this approach is the application of an innovative compressor specifically designed for heat pump operation, together with an optimized refrigerant circuit and control strategy. This concept is expected to significantly outperform the preliminary potential assessment presented here and to deliver a major contribution to the overall efficiency of electric minibuses, particularly at low ambient temperatures.\u003c/p\u003e"},{"header":"Declarations","content":" \u003cp\u003e \u003cstrong\u003eConflict of interest\u003c/strong\u003e \u003cp\u003eThe authors declare no competing interest.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eEthical approval\u003c/strong\u003e \u003cp\u003eNot applicable.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis study was funded by European Union\u0026rsquo;s Horizon research and innovation program under grant number 101138202.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eL.Fiore, C.Beidl, E.Stenger, N.Weimer, N.Kaiser, G.Hohenberg: Testbed setup, methodology, thermal emulators, literature research, test execution, test evaluation and test interpretation, review \u0026ndash; editing. T. B\u0026auml;uml, D. Dvorak: 1D-Multphysical digital twin of the cabin, review \u0026ndash; editing.\u003c/p\u003e\u003ch2\u003eACKNOWLEDGMENT\u003c/h2\u003e \u003cp\u003eThis research was funded by European Union\u0026rsquo;s Horizon research and innovation program under grant number 101138202. The content of this publication is the sole responsibility of the authors and does not necessarily represent the view of the European Commission or its services.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eNo datasets were generated or analysed during the current study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFiore, L., Beidl, C., Stenger, E.: : Development of efficient thermal management systems for HEVs, BEVs and FCEVs using Co-simulation on the HiL-ThermoLab testbed. \u003cem\u003eIn\u003c/em\u003e:: Dritev 2024. VDI, S. 219\u0026ndash;234. 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European Commission (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStephan, P., Schaber, K., Stephan, K.: : Thermodynamik. Springer Berlin Heidelberg, Berlin, Heidelberg, (2013)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePark, M., Kim, S.: : Heating Performance Characteristics of High-Voltage PTC Heater for an Electric Vehicle. \u003cem\u003eIn\u003c/em\u003e: Energies \u003cb\u003e10\u003c/b\u003e Heft 10, S. 1494. (2017). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/en10101494\u003c/span\u003e\u003cspan address=\"10.3390/en10101494\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"automotive-and-engine-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aaet","sideBox":"Learn more about [Automotive and Engine Technology](http://link.springer.com/journal/41104)","snPcode":"41104","submissionUrl":"https://submission.nature.com/new-submission/41104/3","title":"Automotive and Engine Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Energy management, Thermal Management, Co-simulation, HVAC-optimization, Hardware-in-the-Loop testbed, Cabin simulation","lastPublishedDoi":"10.21203/rs.3.rs-8583608/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8583608/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe shift towards sustainable mobility in the passenger- and transport sectors is driving the development of sustainable propulsion technologies. Alongside the advancement of efficient and sustainable powertrains, research increasingly targets the optimization of development processes to address rising system complexity and reduce engineering effort. With the growing adoption of Battery Electric Vehicles (BEVs) and Fuel Cell Electric Vehicles (FCEVs), thermal management is becoming a critical part of the overall vehicle energy management. Considering electric commercial vehicles for public transport, one of the main challenges \u0026ndash; beyond the optimal thermal conditioning of powertrain components \u0026ndash; is the efficient thermal management of the passengers\u0026rsquo; cabin, which significantly impacts overall energy consumption, vehicle range and thermal comfort especially at cold ambient conditions. Particular attention must be paid to the interaction between the HVAC system and the thermal management system of the powertrain components, as can be found in state-of-the-art heat pump systems for electrified vehicles. To enable the early development of operating strategies while accounting for system interactions and determining the optimal system layout, an integrated approach is applied that combines simulation with testing on the Hardware-in-the-Loop (HiL)-ThermoLab testbed. This approach is used in the EU Horizon Europe research project MINDED improving the energy consumption of a battery-electric IVECO eDaily minibus at 0\u0026deg;C ambient temperature. For the development of optimal HVAC layouts and efficient operating strategies, a multi-physical 1D digital twin model of the passenger cabin, described with the modeling language Modelica and validated with measurement results, is used, while all key refrigerant- and thermal circuit components are physically implemented on the testbed. Hardware components that are not yet available are substituted on the HiL-ThermoLab testbed using novel dynamic thermo-hydraulic emulators. The results of this study indicate that the combined use of simulation and hardware testing on the HiL-ThermoLab testbed offers substantial potential for reducing engineering effort in the development process of advanced energy management systems.\u003c/p\u003e","manuscriptTitle":"Efficient development of HVAC systems for electric commercial vehicles through a Hardware-in-the-Loop approach on the ThermoLab testbed","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-11 12:17:37","doi":"10.21203/rs.3.rs-8583608/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-18T16:59:23+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-09T14:55:34+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-17T13:09:29+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"325546857268453882572183349743291684283","date":"2026-02-09T20:23:44+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"316733123537984049193095569619854550891","date":"2026-02-06T10:38:50+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-06T09:54:59+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-22T15:54:22+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-16T04:09:10+00:00","index":"","fulltext":""},{"type":"submitted","content":"Automotive and Engine Technology","date":"2026-01-12T15:37:17+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"automotive-and-engine-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aaet","sideBox":"Learn more about [Automotive and Engine Technology](http://link.springer.com/journal/41104)","snPcode":"41104","submissionUrl":"https://submission.nature.com/new-submission/41104/3","title":"Automotive and Engine Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f4599245-cf03-4b9a-93c6-855dc8df4a71","owner":[],"postedDate":"February 11th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-05T14:39:36+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-11 12:17:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8583608","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8583608","identity":"rs-8583608","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}