## 2 Materials and Methods
### Case Study
#### 2.2.4 Operational Cost Components: Energy, Equipment, and Maintenance  
Electricity and natural gas are the energy sources considered in the analysis. The price of electricity varies depending on whether the energy is taken from the grid or delivered to the grid. In Croatia, the price of electricity for enterprise customers at medium voltage is set at 0.174 \(\upepsilon\)/kWh for the high tariff and 0.105 \(\upepsilon\)/kWh for the low tariff, as of 1 January 2025. The peak power charge is 3.865 \(\upepsilon\)/kW, according to the National distributor for enterprise customers [27]. The price for electricity delivered to the grid is set at 0.125 \(\upepsilon\)/kWh, as per the national legislation [28]. These electricity prices have been adopted for Scenario 1. A comparison of these prices with current prices reveals that at the beginning of 2024, the former was approximately 10% lower. For the second scenario, the electricity prices in effect at that time were adopted, with a high tariff of 0.161 \(\upepsilon\)/kWh, a low tariff of 0.093 \(\upepsilon\)/kWh, and a peak power charge of 3.451 \(\upepsilon\)/kW, by the national distributor for enterprise customers [27]. The price for electricity delivered to the grid is set at 0.114 \(\upepsilon\)/kWh by national legislation [28]. The electricity prices assumed for Scenario 3 are equivalent to those in Scenario 1. Heat energy producers may also have a  
Figure 8: The annual duration of the required heating capacity and energy that can be covered by the operation of the CHP and HP unit with 400 kW heating capacity. different contractual model for electricity pricing, but this scenario is not included in this analysis because of business confidentiality. The price of natural gas has undergone significant fluctuations in recent years. In this paper, the price of natural gas is analysed within three distinct scenarios. The gas price currently applied by the Republic of Croatia is utilised in Scenario 1. The gas price for heat producers is set at the same level as the purchase price, as defined by national guidelines [29]. For Scenario 1, a gas price of 0.0502 \(\upcels\)/kWh was adopted, as reported by the Central European Gas HUB (CEGH) [30]. For Scenario 2, a gas price of 0.030 \(\upcels\)/kWh was implemented, as observed at the beginning of 2024. The third scenario involved a gas price of 0.100 \(\upcels\)/kWh, representing a substantial increase in gas costs. The highest average monthly gas price on the CEGH during 2022 was recorded in August 2022, reaching approximately 0.345 \(\upcels\)/kWh. This surge was driven by supply disruptions and geopolitical tensions linked to the 2022 energy crisis. In order to facilitate the comparison of scenarios, an indicator \(I_{C}\), which represents the electricity-to-gas price ratio, has been introduced. It is calculated using the mean electricity price. \[I_{C}=C_{\text{ee(mean)}}/C_{g} \tag{15}\]  
The HP requires heat from a heat source for its operation. The analysis considered the use of water from a nearby river as a heat source. The fee for using river water is 0.013 \(\upcels\)/m\({}^{3}\). Electricity, gas, and water use prices, as well as corresponding electricity-to-gas price indicators by scenarios, are shown in Table 4.  
Systems with a boiler and CHP unit (B and CHP/B systems) have already been built, so the system's investment will not be considered. In contrast, investment costs for the proposed systems incorporating HP vary depending on the installed capacity. These costs were estimated based on data obtained from equipment manufacturers, including heat pumps, circulation pumps, and piping to the water abstraction point, as well as the associated labour costs for installation. The connection of electrical equipment to the medium-voltage grid operated by the National distributor is classified as a complex interconnection; therefore, the grid connection cost has been incorporated [31].