## 4 Methodology
### Characteristics of the Load and Laboratory Test Facilities  
The HPWH analysed in this work is an Ariston Nuos Evo (80 L of capacity) installed in the laboratory. The main characteristics of the load are shown in Table 2. This appliance has two different heat sources: a heat pump and a resistor. There are three factory settings or modes of service (\"eco\", \"boost\", and \"auto\"). In eco mode, the load only uses the heat pump as a heat source (i.e., to run with a minimum consumption). In boost mode, both heat sources are used simultaneously to reach the target temperature in the minimum time (i.e., the appliance prioritizes service in the short term through maximum consumption). Finally, in auto mode, the HPWH chooses between the heat pump and the resistor to reach the temperature selected in a maximum time of 8 h (i.e., a hybrid mode). This duality of supply and the three different modes presents some problems resulting in inefficient performance. However, a hybrid heat source also presents some opportunities, such as higher flexibility for DR after disabling the factory settings and changing the configuration and control of heat sources. For this objective, the load control was changed externally during our tests. Broad, intrusive monitoring and control systems have been installed in the load and its environment to obtain and tune the parameters of the PBLM. The demonstrator system in the laboratory measures all weather, electrical, thermal, and end-use service variables with relevance to explain load demand and service. To obtain all these variables, a multi-sensor (dwelling temperature and humidity), two temperature sensors (water inlet and outlet) and three thermocouples (inside the reservoir for obtaining hot, mixed, and cold zones temperature) were installed. In addition, two switches manage the switching of each heat source when DR policies are tested for validation. Two external thermostats were also installed to change the control switching of the HP and Resistor, disabling the factory settings of our appliance. Finally, a water flow controller enables the simulation of the discharging cycles of the HPWH with several water draw profiles provided by the control system. Load monitoring and power management were implemented through IP-Symcon platform [27]. This system includes the management of several protocols together (KNX, EnOcean, M-BUS, Modbus, Siemens OZW/S5/S). The measurements of thermocouples are monitored through a data acquisition card linked to the same IP-Symcon platform. The system will be monitored and controlled in the future by an autonomous SymBox Pro unit [27], using different commercial protocols for communication and control that will be analyzed in detail (cost and benefits) for a further potential application to residential segments. Figure 4 presents a scheme of the measurement and control systems installed in the laboratory. Figure 5a shows a picture of the appliance, its environment, and the monitoring system. Note the position of the own temperature (NTC) sensors (Figure 4) for resistors and compressors and the position of intrusive sensors (thermocouples \"S\") installed to validate the model (Figure 5b). Figure 6 presents a plot for some variables that have been monitored by IP-Symcon. As shown, temperatures at different zones in the tank (\"S\" labels in Figure 5b) are similar when the HPWH is near its steady state. In this case, the tank can be modelled as one mass/zone or \"1C\", but temperature stratification appears when the customer demands hot water (in this case, two draws of 31/min for 10 min have been applied by our control system). It can be seen that temperatures at each of the three zones (mass) change with a different pattern. We emphasize that each one drives the two energy conversion devices (in this case, the temperature X\({}_{2}\) drives the start of the heat pump) because the internal sensors of heat sources are at different sites inside the tank (NTC sensors, Figure 5b).