# Feasibility Analysis of Industrial High-Temperature Heat Pumps in Greece
## 3 Results and Discussion
### Parametric Analysis of the Ambient-Source-Driven Unit  
This section discusses the application of the ambient-source HTHP. The present modeling makes the system to have the same performance for all the capacities because the ambient air source mass flow rate is variable. The fluctuation in ambient temperature causes the evaporator temperature to vary accordingly, with a steady temperature difference of 10 K between them. Figure 9 illustrates the variation in COP, and Figure 10 shows the exergy efficiency for different ambient temperatures and process heat temperatures. At high process temperatures (\(\geq\)160 \({}^{\circ}\)C), the system cannot operate at low ambient temperatures, while for 180 \({}^{\circ}\)C process heat, the operation is impossible. Practically, in cases with an extremely high-temperature lift, the compressor cannot operate efficiently and there is a huge need for electricity consumption. Therefore, the operation of the system is not sustainable with COP values being lower than 1. Specifically, operation with a high-temperature lift results in high entropy generation and high superheating in the compressor exit, factors that make this design inefficient thermodynamically. Generally, increases in the ambient temperature make the heat upgrade easier (from ambient temperature level to the process heat temperature level), resulting in increases in COP, as Figure 9 indicates. For the typical ambient temperature of 25 \({}^{\circ}\)C, the maximum COP was found at 2.058 for 100 \({}^{\circ}\)C process heat production, while the minimum value was 1.011 for 170 \({}^{\circ}\)C process heat production. Practically, at high process heat temperatures, the HTHP cannot operate properly, and it is not suggested as a suitable technological solution with the ambient source as the driving source. It is important to note that the COP values for the ambient source are lower compared to the case with the WHS driving. This is expected, as the WHS provides additional exergy input to the system, whereas the ambient source does not add extra exergy to the system. Figure 8: Depiction of Equation (19) accuracy for COP of the WHS-driven HTHP. (The horizontal axis illustrates the COP calculated by the code and the vertical by the approximation equation). Regarding the exergy efficiency that is depicted in Figure 10, the rise in the ambient-source temperature leads to lower exergy efficiency. Practically the higher ambient temperature level makes the process heat production less valuable, a critical fact that reduces the energetic performance. For ambient temperatures up to 30 \({}^{\circ}\)C, the highest exergy efficiency is found for process heat production at 100 \({}^{\circ}\)C, while for higher ambient temperatures, the process heat production at 110 \({}^{\circ}\)C is the optimal case in terms of exergy efficiency. The next step is the development of a regression model for the estimation of the ambient-source HTHP performance in different operating conditions. Therefore, for all the  
Figure 10: Exergy efficiency variation with the ambient temperature for different process heat temperature levels. Figure 9: COP variation with the ambient temperature for different process heat temperature levels. process heat capacities with the ambient-source heat pump, the COP was approximated with the polynomial formula below:  
\[\text{COP}=4.647367-0.00281\cdot\text{T}_{\text{process}}-0.03934\cdot\Delta \text{T}_{\text{lift}}+0.000123\cdot\Delta\Delta_{\text{lift}}^{2} \tag{20}\]  
where the temperature lift in this scenario is defined as \(\Delta\text{T}_{\text{lift}}=\text{T}_{\text{process}}-\text{T}_{\text{am}}\). The regression is significant with R\({}^{2}\) = 99.57%, while the mean COP deviation is 1.39%. Figure 11 indicates that the obtained COP values with the regression model are very close to those initially calculated with the numerical model. The next step is the hourly-based investigation of the air-source HTHP for the city of Athens in Greece.