## 4 Conclusions  
This study examined the viability of integrating medium-scale HPs into existing third-generation DH systems already comprising CHP units and gas boilers. A dynamic simulation model was developed to realistically represent the behaviour of the production, distribution, and consumption subsystems using commercially available technologies. The research assessed multiple system configurations and operational strategies under varying energy price scenarios to determine the energy, environmental, and economic impacts of HP integration.  
The findings demonstrate that incorporating HPs into DH systems can significantly enhance environmental performance. When the HP is given operational priority, systems achieve substantial reductions in primary energy consumption (up to 70%) and CO2 emissions (up to 82%) compared to the baseline configuration with CHP and boiler. Additionally, these configurations result in a higher share of RES, reaching up to 67%, especially with larger HP capacities. However, some reliance on the boiler or CHP remains necessary to cover peak thermal demands.  
Despite these advantages, economic feasibility is highly dependent on the prevailing electricity and gas price ratios. Under current and recent pricing conditions in Croatia (Scenarios 1 and 2), where electricity prices remain high relative to gas, systems with HPs, particularly those with larger capacities, incur higher heating costs and longer payback periods. In these scenarios, configurations prioritising CHP operation remain the most cost-effective due to the opportunity to export electricity. The cost of heating in HP-priority systems can rise by up to 75% under such conditions, making them economically unviable in the short term.  
Conversely, Scenario 3, characterised by more favourable electricity pricing, demonstrates that systems prioritising HP operation can achieve lower levelized costs of heating than the baseline CHP and boiler setup. In this scenario, HP systems exhibit payback periods of 4 to 6 years, with the shortest observed at 4.7 years for a 600 kW HP, confirming the potential for economic viability under supportive market conditions.  
Figure 14: Global costs over the considered period for systems with prioritised CHP operation in price Scenario 3.  
With regard to the generalisation of the results, it is important to note that the economic viability of HP integration depends significantly on local energy prices. Electricity-to-gas price ratios vary widely across Europe, and under conditions of high electricity prices, heat pumps become less economically attractive, while more favourable ratios enhance their competitiveness. In colder climates, the annual thermal energy demand of DH systems would be higher than in the Rijeka case and, conversely, lower in milder climates. However, since the analysed HP uses river water as a heat source, with relatively stable temperatures year-round, its coefficient of performance (COP) remains largely unaffected by ambient air temperature. Although specific national policies (e.g., carbon pricing or subsidies) may influence fuel price dynamics, they would not alter the technical performance of the system. Therefore, while the energy demand and cost-effectiveness may vary across regions, the technical findings remain broadly applicable in similar system configurations.  
In conclusion, the integration of medium-scale HPs into DH systems can offer considerable environmental and energy efficiency benefits, especially when sized to cover a significant portion of the heating load. However, their economic justification is conditional on energy market structures. For widespread adoption, policy support or market reforms that reduce electricity prices relative to gas, alongside technical measures such as lowering system supply temperatures, will be essential. Further optimisation of DH system configurations and the adoption of low-temperature networks could enhance both the efficiency and cost-effectiveness of HP integration in future DH systems.